Memory problem, Vista problem, or something else?

OK, so a couple of months ago, I had a motherboard burn out on me. Fought with Intel and got a new one, only now when I turn on my machine, Windows is r-e-a-l-l-l-l-l-y slow to load, and I can't run programs of any significance without the machine freezing halfway through the startup sequence. This is a machine that was ridiculously overpowered for what I was asking it to do; it used to play Crysis without even breaking a sweat.

Seemed like there was something wrong with the memory, so I tested out each individual stick (found nothing), then finally replaced all my 4GB of RAM with new sticks. Still having the exact same problem.

So at this point, seems like the RAM is ruled out. It seems pretty unlikely that it's a problem with my fricking BRAND NEW motherboard, so my next step is probably to wipe the whole thing and reinstall Windows. Unless it's something I haven't thought of -- could there be a problem with the BIOS or something? This is why I'm asking here first for any ideas before trying something that drastic.

Intel Q9550 2.93 gHz
8GB RAM (Patriot DDR3 7-7-7-20)
Intel DP45SG motherboard
NCTZ 850MW power supply
Radeon HD4870 512MB

basically ... WTF?

I always do a format and fresh installation of windows with each motherboard change, especially with different chipsets. Some folks work around this, but you have to be pretty sharp with software. If necessary, backup your files first before formating.