diff -r aff959a3ba13 Lib/idlelib/AutoComplete.py
--- a/Lib/idlelib/AutoComplete.py	Sun Sep 08 18:44:40 2013 -0500
+++ b/Lib/idlelib/AutoComplete.py	Tue Sep 10 00:36:53 2013 -0700
@@ -216,7 +216,13 @@
                     expandedpath = os.path.expanduser(what)
                     bigl = os.listdir(expandedpath)
                     bigl.sort()
-                    smalll = [s for s in bigl if s[:1] != '.']
+                    # Handle case-insensitivity on Windows
+                    if os.name == 'nt':
+                        smalll = [s.lower() if s.lower() not in bigl else s
+                                  for s in bigl if s[:1] != '.']
+                    else:
+                        smalll = [s for s in bigl if s[:1] != '.']
+                        # XXX: Not sure why this is s[:1] instead of s[0]
                 except OSError:
                     return [], []
 
diff -r aff959a3ba13 Lib/idlelib/AutoCompleteWindow.py
--- a/Lib/idlelib/AutoCompleteWindow.py	Sun Sep 08 18:44:40 2013 -0500
+++ b/Lib/idlelib/AutoCompleteWindow.py	Tue Sep 10 00:36:53 2013 -0700
@@ -1,6 +1,7 @@
 """
 An auto-completion window for IDLE, used by the AutoComplete extension
 """
+import os
 from tkinter import *
 from idlelib.MultiCall import MC_SHIFT
 from idlelib.AutoComplete import COMPLETE_FILES, COMPLETE_ATTRIBUTES
@@ -99,7 +100,14 @@
         last = i-1
 
         if first == last: # only one possible completion
-            return self.completions[first]
+            if os.name == 'nt':
+                i = 0
+                for s in map(str.lower, self.ntcompletions):
+                    if s == self.completions[first]:
+                        return self.ntcompletions[i]
+                    i += 1
+            else:
+                return self.completions[first]
 
         # We should return the maximum prefix of first and last
         first_comp = self.completions[first]
@@ -152,6 +160,8 @@
         matching completion, don't open a list."""
         # Handle the start we already have
         self.completions, self.morecompletions = comp_lists
+        if os.name == 'nt':
+            self.ntcompletions = self.morecompletions
         self.mode = mode
         self.startindex = self.widget.index(index)
         self.start = self.widget.get(self.startindex, "insert")