With this command you are trying to run a file virtualenvin a folder C:\. This is not what you want.
I do not know how you installed the virtualenvWindows window, but the installation script should have been placed virtualenv.exein C:/path/to/python/Scripts( C:/Python27/Scripts/virtualenv.exeon my machine).
So all you need is
C:\>C:/path/to/python/Scripts/virtualenv.exe flask
PS: Put C:/path/to/python/ScriptsinPATH
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