So I have had a look at this post and am still somewhat in the mire. My computer is:
CM Stack Case, Tagan 900W PSU, Gigabyte K8NSNXP, 2 Gig Corsair something or other RAM, AMD 64 X2 4400+, 3 HDD (2x 200GB and 1x 120GB), ATI HD 3850 AGP.
It has moved house recently an then sat unused for 3 months in a garage. Since then I took it out and set it going. Was running windows update when it shutdown on its own. Turned back on and pulled some plugs out of the bank thinking I had put too much load through the multiplug. Same result. Installed speedfan and was just in time to see the temp rise to just under 100C!!! (on the Core variable, which I believe is the CPU from previous experience with Speedfan on this mobo).
Decided it must be the heatsink. spent an age degunking it from a hefty layer of dust, likewise for the other fans. Took it off, cleaned of paste and reapplied/reseated it. All seemed to be going well last night doing a virus scan and updates. Today however the problem has reappeared. I am worried it might be something more insipid than just poor thermal contact between the cooler and chip. Could the chip be damaged and thus overheating or am i jumping to conclusion? Any thoughts greatly appreciated.