Python install puts all files/libs in c: root directory (Windows 7 64-bit)
Did you specify C:\Python25 as the install directory in the GUI installer? It should respect that setting. If not, you can specify a target install directory from the command line like this:
msiexec /i python-2.5.amd64.msi TARGETDIR=c:\python25
Ensure you are in the directory containing the msi when you run the command, and that command prompt has been run as administrator.