Add missing label.
diff --git a/Doc/using/windows.rst b/Doc/using/windows.rst
index 978440c..b49ba7b 100644
--- a/Doc/using/windows.rst
+++ b/Doc/using/windows.rst
@@ -84,6 +84,8 @@
 settings in Windows.
 
 
+.. _setting-envvars:
+
 Excursus: Setting environment variables
 ---------------------------------------