Yes, strange fix. Typically you cannot upgrade from 32-bit to 64-bit without doing a clean installation. But, thanks for posting, hopefully this will help someone else as well
Just wanted to let anyone know who may be struggling with this...
I tried to install windows 7 twice, upgrading from XP 32 bit to 7 64 bit and had issues. After it was all installed I just got a black screen. Tried to start in safe mode and it said "cannot complete installation in safe mode" and restarted.
Anyway I ended up just unplugging one of my monitor connections leaving just the VGA plug in and it worked perfectly. Kind of a strange fix but in case you're having the same problem, there you go!