BSOD and/or connstant rebooting. Mobo issue?

Hello. For the past few weeks I've been experiencing issues with my homebuilt system. Here are my specs:

Intel Pentium 4 2.8 processor
GeForce 6800 GT 256MB video card
400GB Seagate IDE hard drive
4x500MB DDR2 PC3200 RAM
ASUS P4P800 Motherboard

That should be about good for relevency.

Anyway, initially, the issue was Windows XP freezing and/or rebooting. I thought it may have been an issue with Windows, so decided to try reinstalling. This is where things went screwy on me. I cannot reinstall it. It reboots or blue screens(generic BSOD DRIVER_NOT_LESS_EQUAL error, no other info).

I've tried reinstalling under the following conditions in an effort to eliminate certain pieces of hardware:

1) With each stick of RAM individually
2) A different hard drive
3) A different video card
4) A different CD-ROM

All with the same results as before. I am leaning towards a motherboard or cpu issue, but would like your thoughts/ideas on the situation. Is there a way to determine exactly what the cause is?