My thoughts.
1. The heatsink is not installed proper.
http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_869_4348%5E6678,00.html
2. Try slowing the machine down... Find a BIOS FSB setting or jumper and go from 200 to 166 or 133 to 100 MHz. Slow down until you find it.
3. An overheated CPU can "fry". Prepare yourself mentally to replace the CPU.
4. Look for domed and/or leaking motherboard capacitors.
Bob
PS. I know you asked what could have caused this, but I think you are wanting some ideas on how to recover.
Today I changed my CPU fan because it was making a really loud noise and the computer overheated and restarted after about 5 minutes of use.
After I put in the new fan/heat sink my computer will not boot up past the Windows logo screen. The bar below the logo moves for less than a second and the computer just restarts. No error message appears. Safe mode does not work either.
Computer is using:
Asus A7v8x-x motherboard
AMD Athlon XP 2600+
512 MB DDR RAM
Radeon 9000 128 MB
What could have caused this? Please help.

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