I vaguely remember something about problems with 32bit to 64bit upgrades from my Win 7 DST course.
IIRC, The workaround is to install from scratch and then perform an 'upgrade' on the newly installed OS.
http://windows.microsoft.com/en-US/wind ... -questions
If you want to move from a 32-bit version of Windows to a 64-bit version of Windows 7 or vice versa, you'll need to back up your files and choose the Custom option during Windows 7 installation. Then, you'll need to restore your files and reinstall your programs. For more information about performing a custom installation, see Installing and reinstalling Windows 7.
Also see: http://windows.microsoft.com/en-US/wind ... -Windows-7
for a step by step guide.