diff --git a/Doc/lib/libexcs.tex b/Doc/lib/libexcs.tex
index f52ff0a..b51b7fc 100644
--- a/Doc/lib/libexcs.tex
+++ b/Doc/lib/libexcs.tex
@@ -399,11 +399,15 @@
 \begin{excdesc}{WindowsError}
   Raised when a Windows-specific error occurs or when the error number
   does not correspond to an \cdata{errno} value.  The
-  \member{errno} and \member{strerror} values are created from the
+  \member{winerror} and \member{strerror} values are created from the
   return values of the \cfunction{GetLastError()} and
   \cfunction{FormatMessage()} functions from the Windows Platform API.
+  The \member{errno} value maps the \member{winerror} value to 
+  corresponding \code{errno.h} values.
   This is a subclass of \exception{OSError}.
 \versionadded{2.0}
+\versionchanged[Previous versions put the \cfunction{GetLastError()}
+codes into \member{errno}]{2.5}
 \end{excdesc}
 
 \begin{excdesc}{ZeroDivisionError}
diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 2d3553a..bb115e1 100755
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -24,12 +24,6 @@
             'BabylMessage', 'MMDFMessage', 'UnixMailbox',
             'PortableUnixMailbox', 'MmdfMailbox', 'MHMailbox', 'BabylMailbox' ]
 
-if sys.platform != 'win32':
-    # Define WindowsError so that we can use it in an except statement
-    # even on non-Windows systems
-    class WindowsError:
-        pass
-
 class Mailbox:
     """A group of messages in a particular place."""
 
@@ -268,9 +262,6 @@
             self.remove(key)
         except KeyError:
             pass
-        except WindowsError, e:
-            if e.errno != 2: # ERROR_FILE_NOT_FOUND
-                raise
         except OSError, e:
             if e.errno != errno.ENOENT:
                 raise
@@ -426,12 +417,6 @@
         path = os.path.join(self._path, 'tmp', uniq)
         try:
             os.stat(path)
-        except WindowsError, e:
-            if e.errno == 2: # ERROR_FILE_NOT_FOUND
-                Maildir._count += 1
-                return open(path, 'wb+')
-            else:
-                raise
         except OSError, e:
             if e.errno == errno.ENOENT:
                 Maildir._count += 1
@@ -579,12 +564,6 @@
         self._file.close()
         try:
             os.rename(new_file.name, self._path)
-        except WindowsError, e:
-            if e.errno == 183: # ERROR_ALREADY_EXISTS
-                os.remove(self._path)
-                os.rename(new_file.name, self._path)
-            else:
-                raise
         except OSError, e:
             if e.errno == errno.EEXIST:
                 os.remove(self._path)
@@ -1856,13 +1835,6 @@
                 else:
                     os.rename(pre_lock.name, f.name + '.lock')
                     dotlock_done = True
-            except WindowsError, e:
-                if e.errno == 183:  # ERROR_ALREADY_EXISTS
-                    os.remove(pre_lock.name)
-                    raise ExternalClashError('dot lock unavailable: %s' %
-                                             f.name)
-                else:
-                    raise
             except OSError, e:
                 if e.errno == errno.EEXIST:
                     os.remove(pre_lock.name)
diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index 83dfa17..dd7e864 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -327,10 +327,6 @@
         try:
             _os.mkdir(file, 0700)
             return file
-        except WindowsError, e:
-            if e.errno == 183: # ERROR_ALREADY_EXISTS
-                continue # try again
-            raise
         except OSError, e:
             if e.errno == _errno.EEXIST:
                 continue # try again
diff --git a/Misc/NEWS b/Misc/NEWS
index 4e1aa9e..0a769b7 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,11 @@
 Core and builtins
 -----------------
 
+- WindowsError now has two error code attributes: errno, which carries
+  the error values from errno.h, and winerror, which carries the error
+  values from winerror.h. Previous versions put the winerror.h values
+  (from GetLastError()) into the errno attribute.
+
 - Patch #1475845: Raise IndentationError for unexpected indent.
 
 - Patch #1479181: split open() and file() from being aliases for each other.
diff --git a/PC/errmap.h b/PC/errmap.h
new file mode 100644
index 0000000..59aeea1
--- /dev/null
+++ b/PC/errmap.h
@@ -0,0 +1,78 @@
+/* Generated file. Do not edit. */
+int winerror_to_errno(int winerror)
+{
+	switch(winerror) {
+		case 2: return 2;
+		case 3: return 2;
+		case 4: return 24;
+		case 5: return 13;
+		case 6: return 9;
+		case 7: return 12;
+		case 8: return 12;
+		case 9: return 12;
+		case 10: return 7;
+		case 11: return 8;
+		case 15: return 2;
+		case 16: return 13;
+		case 17: return 18;
+		case 18: return 2;
+		case 19: return 13;
+		case 20: return 13;
+		case 21: return 13;
+		case 22: return 13;
+		case 23: return 13;
+		case 24: return 13;
+		case 25: return 13;
+		case 26: return 13;
+		case 27: return 13;
+		case 28: return 13;
+		case 29: return 13;
+		case 30: return 13;
+		case 31: return 13;
+		case 32: return 13;
+		case 33: return 13;
+		case 34: return 13;
+		case 35: return 13;
+		case 36: return 13;
+		case 53: return 2;
+		case 65: return 13;
+		case 67: return 2;
+		case 80: return 17;
+		case 82: return 13;
+		case 83: return 13;
+		case 89: return 11;
+		case 108: return 13;
+		case 109: return 32;
+		case 112: return 28;
+		case 114: return 9;
+		case 128: return 10;
+		case 129: return 10;
+		case 130: return 9;
+		case 132: return 13;
+		case 145: return 41;
+		case 158: return 13;
+		case 161: return 2;
+		case 164: return 11;
+		case 167: return 13;
+		case 183: return 17;
+		case 188: return 8;
+		case 189: return 8;
+		case 190: return 8;
+		case 191: return 8;
+		case 192: return 8;
+		case 193: return 8;
+		case 194: return 8;
+		case 195: return 8;
+		case 196: return 8;
+		case 197: return 8;
+		case 198: return 8;
+		case 199: return 8;
+		case 200: return 8;
+		case 201: return 8;
+		case 202: return 8;
+		case 206: return 2;
+		case 215: return 11;
+		case 1816: return 12;
+		default: return EINVAL;
+	}
+}
diff --git a/PC/errmap.mak b/PC/errmap.mak
new file mode 100644
index 0000000..646bcd0
--- /dev/null
+++ b/PC/errmap.mak
@@ -0,0 +1,5 @@
+errmap.h:	generrmap.exe
+	.\generrmap.exe > errmap.h
+
+genermap.exe:	generrmap.c
+	cl generrmap.c
diff --git a/PC/generrmap.c b/PC/generrmap.c
new file mode 100644
index 0000000..2b25063
--- /dev/null
+++ b/PC/generrmap.c
@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include <errno.h>
+
+/* Extract the mapping of Win32 error codes to errno */
+
+int main()
+{
+	int i;
+	printf("/* Generated file. Do not edit. */\n");
+	printf("int winerror_to_errno(int winerror)\n");
+	printf("{\n\tswitch(winerror) {\n");
+	for(i=1; i < 65000; i++) {
+		_dosmaperr(i);
+		if (errno == EINVAL)
+			continue;
+		printf("\t\tcase %d: return %d;\n", i, errno);
+	}
+	printf("\t\tdefault: return EINVAL;\n");
+	printf("\t}\n}\n");
+}
diff --git a/Python/exceptions.c b/Python/exceptions.c
index 31fb53e..3f949ed 100644
--- a/Python/exceptions.c
+++ b/Python/exceptions.c
@@ -704,15 +704,132 @@
     {NULL, NULL}
 };
 
-
-
-
 PyDoc_STRVAR(IOError__doc__, "I/O operation failed.");
 
 PyDoc_STRVAR(OSError__doc__, "OS system call failed.");
 
 #ifdef MS_WINDOWS
+#include "errmap.h"
+
 PyDoc_STRVAR(WindowsError__doc__, "MS-Windows OS system call failed.");
+
+static PyObject *
+WindowsError__init__(PyObject *self, PyObject *args)
+{
+	PyObject *o_errcode, *result;
+	long errcode, posix_errno;
+	result = EnvironmentError__init__(self, args);
+	if (!result)
+		return NULL;
+	self = get_self(args);
+	if (!self)
+		goto failed;
+	/* Set errno to the POSIX errno, and winerror to the Win32
+	   error code. */
+	o_errcode = PyObject_GetAttrString(self, "errno");
+	if (!o_errcode)
+		goto failed;
+	errcode = PyInt_AsLong(o_errcode);
+	if (!errcode == -1 && PyErr_Occurred())
+		goto failed;
+	posix_errno = winerror_to_errno(errcode);
+	if (PyObject_SetAttrString(self, "winerror", o_errcode) < 0)
+		goto failed;
+	Py_DECREF(o_errcode);
+	o_errcode = PyInt_FromLong(posix_errno);
+	if (!o_errcode)
+		goto failed;
+	if (PyObject_SetAttrString(self, "errno", o_errcode) < 0)
+		goto failed;
+	Py_DECREF(o_errcode);
+	return result;
+failed:
+	/* Could not set errno. */
+	Py_XDECREF(o_errcode);
+	Py_DECREF(self);
+	Py_DECREF(result);
+	return NULL;
+}
+
+static PyObject *
+WindowsError__str__(PyObject *self, PyObject *args)
+{
+    PyObject *originalself = self;
+    PyObject *filename;
+    PyObject *serrno;
+    PyObject *strerror;
+    PyObject *rtnval = NULL;
+
+    if (!PyArg_ParseTuple(args, "O:__str__", &self))
+	return NULL;
+
+    filename = PyObject_GetAttrString(self, "filename");
+    serrno = PyObject_GetAttrString(self, "winerror");
+    strerror = PyObject_GetAttrString(self, "strerror");
+    if (!filename || !serrno || !strerror)
+	goto finally;
+
+    if (filename != Py_None) {
+	PyObject *fmt = PyString_FromString("[Error %s] %s: %s");
+	PyObject *repr = PyObject_Repr(filename);
+	PyObject *tuple = PyTuple_New(3);
+
+	if (!fmt || !repr || !tuple) {
+	    Py_XDECREF(fmt);
+	    Py_XDECREF(repr);
+	    Py_XDECREF(tuple);
+	    goto finally;
+	}
+
+	PyTuple_SET_ITEM(tuple, 0, serrno);
+	PyTuple_SET_ITEM(tuple, 1, strerror);
+	PyTuple_SET_ITEM(tuple, 2, repr);
+
+	rtnval = PyString_Format(fmt, tuple);
+
+	Py_DECREF(fmt);
+	Py_DECREF(tuple);
+	/* already freed because tuple owned only reference */
+	serrno = NULL;
+	strerror = NULL;
+    }
+    else if (PyObject_IsTrue(serrno) && PyObject_IsTrue(strerror)) {
+	PyObject *fmt = PyString_FromString("[Error %s] %s");
+	PyObject *tuple = PyTuple_New(2);
+
+	if (!fmt || !tuple) {
+	    Py_XDECREF(fmt);
+	    Py_XDECREF(tuple);
+	    goto finally;
+	}
+
+	PyTuple_SET_ITEM(tuple, 0, serrno);
+	PyTuple_SET_ITEM(tuple, 1, strerror);
+
+	rtnval = PyString_Format(fmt, tuple);
+
+	Py_DECREF(fmt);
+	Py_DECREF(tuple);
+	/* already freed because tuple owned only reference */
+	serrno = NULL;
+	strerror = NULL;
+    }
+    else
+	rtnval = EnvironmentError__str__(originalself, args);
+
+  finally:
+    Py_XDECREF(filename);
+    Py_XDECREF(serrno);
+    Py_XDECREF(strerror);
+    return rtnval;
+}
+
+static
+PyMethodDef WindowsError_methods[] = {
+    {"__init__", WindowsError__init__, METH_VARARGS},
+    {"__str__", WindowsError__str__, METH_VARARGS},
+    {NULL, NULL}
+};
 #endif /* MS_WINDOWS */
 
 #ifdef __VMS
@@ -1760,7 +1877,7 @@
  {"OSError", &PyExc_OSError, &PyExc_EnvironmentError, OSError__doc__},
 #ifdef MS_WINDOWS
  {"WindowsError", &PyExc_WindowsError, &PyExc_OSError,
-  WindowsError__doc__},
+WindowsError__doc__, WindowsError_methods},
 #endif /* MS_WINDOWS */
 #ifdef __VMS
  {"VMSError", &PyExc_VMSError, &PyExc_OSError,