Had problems with XP SP3, original problem was a virus, reformatted HD, Reloaded XP performed all the required updates machine ran fine until I asked it to do any labor intensive work. Things ran then would re boot, would then boot up funny, screen freezing etc. looking like another virus at times. Because I was working with replacing a lot of software assumed it was software related, even blamed my printer software as that was being flagged in the event viewer.
Someone here had mentioned a hardware possibility and possibly heat related. I remembered having similar problems a long while ago with another machine. I checked my heat sink apparatus for problems and the restraining carriage fell apart in my hands. The plastic due to age and heat became brittle and was severely cracked. The Heat sink was making enough contact that the CPU was not heating during low work apps but when you went to do work it over heated. Looking at the heat paste it was dry and indicating that it was barely doing its function.
I rebuilt the heat sink restraint, re-pasted and tested and thus far all is running perfect.
I am not saying this will solve for everyone the problemss but for me it was hardware, and in particular the heat sink. Something to check, even if it looks to be fine.