This bug appears (for me) when I use longer install paths, both in the GUI installer and doing a silent install via msiexec. The paths I've used so far are these:

E:\langs\Python\x32\27
E:\langs\Python\x32\34
E:\langs\Python\x64\27
E:\langs\Python\x64\34

E:\Python\x32\27
E:\Python\x32\34
E:\Python\x64\27
E:\Python\x64\34

Both of these paths fail to install when pip is called. If I install to a single-level directory, the install works fine (doesn't matter what drive I use either....)