Alright, as some of you know, I have an old computer in the very-very low range. It was a Celeron 850MHz, 512MB SD100 (two sticks were 133) RAM, 20/40GB HDD @ 5400/7200RPM respectively, GeForce FX 5200, 52x CD, and something like a 250w power supply (had roughly half the ratings of a cheap 550w I bought from staples, returning it because the -5v rail is -6.15v! and the 5v is 4.79 or something like that), crappy little 15" CRT monitor with built in speakers and mic, etc.

Well, its gone through a hell of a week of upgrades. I've spent a good $150 on it now. It's still below low-end spec of today, running an AMD Athlon XP 1600+ Palomino core and an ASUS K7V-133 (KT133a chip). 896MB of PC133 (yeah, all 3 are SD133 now) RAM, same harddrives, GeForce 6600GT, Sound Blaster Audigy 2 ZS (exchanged the Audigy 2 for this one, due to all the crackling I got ), and an LG Super-multi DVD burner w/o lightscribe (takes a rediculously long time, a print labeller is by far faster in the end, and it's pretty cheap), 17" LG flatron LCD monitor (on DVI), Logitech X-540 TRUE 5.1 Surround, Logitech G11 keyboard and Microsoft comfort 2000dpi laser mouse. Same PSU and video

Now, it worked perfectly fine before I got the sound card, but it may also be driver corruption (Doom 3 problems; driver confictions; crappy Creative Drivers not working ending up corrupting my harddrive, etc.) There are only 2 things I can think of that are causing these random lockups. #1: My power supply, it's not as frequent with the newer one, but I really dont like the -5v being that far off. Or #2 EAX in gaming. Windows runs perfectly fine until a game or movie starts (i was watching Are we Done Yet? in 5.1 and it locked up like 45 minutes in. NFS Most Wanted, doesnt even have surround, but it locks up when anything 3D shows up) Windows is still on ACPI and I removed the new power supply before reinstalling windows. Before the reinstall, not only did it have ACPI disabled (so IRQ's wouldn't mix 'n' match), but it had a bigger PSU

Now, here are the things I've tried to solve this:
tweaked a couple bios settings, minor things like PCI delayed transaction, Disable RAID, etc.
swapping power supplies (250w to 550w and back)
Yeah, I've tried the RAM
reinstalling windows
beating it with a stick
disconnecting a harddrive and floppy
additional cooling fan
yelling at it (mostly cursing )

This is just getting retarded...

Check the heat sink your GPU...

most times this can be traced to bad vid drivers , been there

it stopped locking up (mostly, still does the odd time), grabbing a bigger PSU and new fan for both chipset and CPU just to be sure those arent the problems. This thing runs hot already, may as well help it cool off. I used old drivers (from BFG CD 4.4 comes with the 6600GT AGP), new drivers (162.xx), and betas (163.xx). All 3 are stable. I THINK the problem was a bad windows CD (another crapped out custom edition, oh well, used an older good-version I made like last year).