Sounds like a virus causing the sudden restart. Had similar on my PC. I did a reinstall like you but problem did come back eventually and was the same virus. Could be a corrupt windows and its bombing out when referencing certain memory. Could be a faulty hard disk with bad sectors and only bombs out when reading this. Does it seem to happen when you are doing the same tasks or viewing the same folders or is it random? Do you get any warning before it shuts down or is it instant ie does it actually shutdown or just crash down? Can you get into safe more or boot into last known good configuration by pressing F8 at startup? This happened to me prior to installing SP2.