I think it is to do with the Nforce 3 chipset.
You should be able to still bootup in safe-mode at least.
go to c\windows\system 32\drivers
locate the atikmdag.sys file and rename it (change extenstion to .doc or .cbf is prob best)
then reboot into ordinary mode...
BTW this is dangerous. I am fairly sure this is either the power management driver or something, but vista *should* compensate. And remember safe mode!
Windows should not BSOD then, I won't say your computer will be 100% stable, but it should run.
AFAIK the atikmdag.sys problem is a mixture of AGP, Nforce 3 and Vista...
Before doing this though, try rebooting with no USB devices attatched. Weird and wonderful as it may seem that could be the problem.