Here's the relevant bit of the output from that buildbot after Terry's change:

======================================================================
FAIL: testLoadWithUNC (test.test_tcl.TclTest)
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Traceback (most recent call last):
  File "E:\Data\buildslave\cpython\2.7.snakebite-win2k3r2sp2-x86\build\lib\test\test_tcl.py", line 151, in testLoadWithUNC
    self.assertIn('Tkinter.py', f.read())
AssertionError: 'Tkinter.py' not found in ''
----------------------------------------------------------------------

...which doesn't say much.  Here's a patch that updates the test to use subprocess.Popen instead of os.popen and add some useful debugging information.  The test passes on my machine with and without the patch, so I can't do much more debugging until we get some useful output from the buildbot.