The reason why I asked for a description is because there are 2 general types of reboot problems:
(1) Manual
(2) Automatic
So if yours is an automatic problem, then in addition to technome's suggestions, it can also be caused by the following:
-Everything not completely seated on the motherboard.
-Overheating
If the system temp gets over 45 deg. C, then make sure the computer's fans are running, the vents are open, the computer is not located in an enclosed space or near a heat source, and there is no internal dust build up.
If those are ok, then remove the computer's cover, and run an external fan. If this fixes things, then install additional case fans. Recommended locations are lower front and upper rear of the case.
If the CPU gets over 60 deg. C, then make sure there is thermal compound between it and the heat sink. If that's ok, then replace the compound with Arctic Silver3.
If no luck, then install a higher capacity heat sink fan.
If still no luck, then replace the heatsink with a more efficient unit.
-Bad video card.
Make sure it's fully seated, it's fan is running, and its heatsink is securely mounted.
If those are ok, then swap it out with a known good card.
-Bad MB.
Check for bulging, leaking, or ruptured capacitors.
If they look ok, then swap it out with a known good MB.
-IRQ conflict with a network card.
If no problems show up for the NIC in Device Manager, then remove it from DM, shut down, uninstall the card, and restart.
-Viruses and spyware.
-Kazaa.
