[MetalMilitia]

The CPU isn't a downgrade because a newer processor has a more efficient architecture thus getting more "work" done in less cycles. It's like now the old Athlon XP series needed a naming scheme that informed the consumer that they were equivilient to higher clocked P4s.

If you built the machine yourself it could be anything - but probably something to do with your graphics/sound/motherboard drivers. Could also be a memory problem.

Go to "My Computer -> Properties -> Advanced system settings -> Advanced -> Startup and recovery settings" and disable "automatically restart". Then load up Portal and if it displays a BSOD make a note of the EXE that caused the crash. Now reboot the PC and google the EXE - you should be able to find out which software caused the crash.