My system is restarting very frequently after useage of few hours.
My config is E7400 2.4 Ghz. ASUS P5K-PL CM (MotherBoard). 2GB RAM.
WIN XP and Norton Internet 2010 installed.
System is in an airconditioned room.So i don't think over heating is an issue.
Might be a simple case of some wires gone loose inside the cabinat. coincidentally, I've fixed a similar problem today itself in one of my uncle's computer. The system would suddenly shutdown. I just opened the cabinet and tightened all the cables and the problem was gone. be careful though, don't get a shock or damaged part.
check your ram slots and other cables. they might be loose.
i had that problem when my PCs hard reset button right below the main power button got faulty. it used to close the circuit by itself so i used to get frequent system restarts . i replaced it with a new circuit and its working fine now.
you can check the temperatures using speccy. 47 is a bit high in an AC room, I guess You checked it when the AC was off. If not, check the CPU fan & other fans and clean the inside of the cabinet carefully.