Just recently I had a bunch of crashes (STOP 0xC4) in my Vista Ultimate 64 bit system. I traced it to Microsoft's Virtual PC. Since it had been running just fine for several months, I thought it may have been a Windows Update. Diver verifier pointed to the Virtual Machine's drivers - so I was stumped!!!

Some fixes suggested were:
- update the BIOS
- Turn off hardware virtualization in the Virtual PC console
- don't use the 32 bit version on 64 bit Vista

I wasn't able to stay in Virtual PC long enough to adjust the settings, the BIOS has been rock stable since day 1 - so I hesitated updating it.

Being the know it all that I am, I never checked to see which version of Virtual PC that I had (it's simple to check, just look for it in Program Files (for 64 bit) or Program Files (x86) for the 32 bit.

So, I uninstalled it, downloaded another copy and installed that. Still crashed!!! Drat!!!
Checked the Program Files folder and it wasn't there - found it in the Program Files (x86) folder - What The Heck!?!?!?
Uninstalled it, downloaded the 64 bit version and installed it - no crashes!!!

All I can figure is that I had originally installed the 32 bit version - and it worked well with that until some unknown Windows Update came along and broke it. Installing the 64 bit version seems to have solved all of my problems.

Just posting this in the event that someone else runs into this problem - it stumped me for a couple of days!