Pull request #84 broke tox for Windows, because virtualenv is not added into %PATH%. Looking up for virtualenv through interpreter is a way to go. At least this way should have been left as a fallback.
fix issue158 and fix issue155: windows/virtualenv properly works now:
call virtualenv through "python -m virtualenv" with the same
interpreter which invoked tox. Thanks Chris Withers, Ionel Maries Cristian.