Hardlocks in Windows XP?

I get frequent hardlocks in XP, and I've tried every thing I can think of. Individual RAM testing shows that the RAM itself is fine, I removed all hardware (and tried 2 different video cards) to no avail. Now I just reformatted and I still get the hardlocks.

They occur whenever I'm downloading a file or strain is being put on the processor.

This leads me to believe it is a processor error, any way I can test this hypothesis?

And of course, reseat the CPU fan/heatsink with a new application of thermal grease - if the thermal interface material has dried out it won't conduct heat to the heatsink properly.