My problem was an overheating CPU. I think stock came running at 40C-45C load, last year or so has been running 52C-59C load, and around 66C it will cause the computer to reboot itself.
Well I was getting alot of reboots. So I just said hell, I'll replace the heatsink/fan and see if that resolves the issue. Installation was tricky since one of the pieces that lock the heatsink in an AMD was broken and stuck, so I had to lodge the piece of plastic off with a screwdriver to lock in the heatsink. That was the only "rough" part.
Now when I start my computer it does one of many things:
Error 1: It will just reboot itself during any point of the load process.
Error 2 (most common error): During Windows 2000 load, about midway gives me a KMODE EXCEPTION NOT HANDLED blue screen of death.
Error 3: I sometimes get other bluescreens. I even get them with boot discs, with Windows 2K boot disc I get KMODE. With a Win XP boot disc I get a different error "BAD_POOL_CALLER." Doesn't seem to be related to my hard drive.
I tried resetting my BIOS, replacing the lithium battery, and using the default setting + changing the settings randomly many times. Still won't boot.
I tried running MemTest and different Knoppix cds, it just crashes while it is loading any of them- no error given, just a total freeze.
I figure faulty memory might be to blame, but I swapped in and out many sticks of memory to no difference in the error types. Occasionally my computer doesn't even recognize the memory and gives me the three beeps of doom. So I really can't see any other issue than that I have to replace my mobo.
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So here we go. Old computer hardware (3-4 years old), a seemingly simple replacement ended up totally blowing up my computer to a problem I can't seem to diagnose. I'm at the point of just retiring it and buying a new computer, but I really didn't think replacing the heatsink/fan would have killed my computer.
Maybe you guys have some ideas of something I haven't tried. I can't load any boot discs without getting the errors, and the floppy drive stopped working two years ago. I doubt it is a hard drive issue or virus since I had virtually no problems with it, except the overheating, before I replaced the heatsink. Plus, if the hard drive were culprit a bootable CD like Knoppix should work.
My next idea might be to upgrade the BIOS, though I'm not sure how I'm going to do that when nothing seems to load or work.
I probably can get a decent tower for around $300 so I probably will just ignore this headache and go for that. I'm just mad that I've finally found a computer I can't repair of the thousands I have repaired through college, my own POS.

EDIT:
Ah and it does the same thing when booting safe mode or any of the other booting options. But the at least the CPU temperature seems stable and much better than it was before, so I don't think I messed up there.