With Python 2.7.10 and virtualenv 14.0.2 or 14.0.3, "virtualenv env" fails on Windows with "ImportError: No module named UserDict". This is presumably due to the fact it installs userdict.py rather than UserDict.py in env\lib.
This worked on 14.0.1.
Workaround: manually running "ren env\lib\userdict.py UserDict.py" seems to fix the created virtualenv.
I'd guess it's the "filename = os.path.normcase(os.path.abspath(filename))" line added in ac4ea65.
Revert ac4ea65; only correct drive letter case.
Fixes #856, #815