Have a fix for this that I'll send off shortly.

What I see with the current head (my username was replaced with "Foo Bar" in this example:
> Using "C:\Users\Foo Bar\cpython\PCbuild\\..\externals\pythonx86\tools\python.exe" (found in externals directory)
> Bar\cpython\PCbuild\\..\externals\pythonx86\tools\python.exe""=="" was unexpected at this time.

My theory, window's turning:
> C:\Users\Foo Bar
into
> "C:\Users\Foo Bar"
and this is colliding with our use of "%PYTHON%", creating double quotes, or:
> ""C:\Users\Foo Bar""
which, of course:
> if ""C:\Users\Foo Bar""==""
does not make sense as a statement.