Originally Posted by FastFordChris
if its rebooting while playing games etc then its most likely going to be a overheating issue, as thats the pc's fail safe once it hits a certain temperature.
get yourself a compressed air spray can and clean your case/fans/ everything out, if your graphics card has a fan then clean all that out as well.
this is well worth doing on a regular basis as its a major cause of over heating.
I'd go with this as well definately heat related either CPU or GFX card woudn't have though Memory tbh.
After you have cleaned all your fans id check that the heatsinks are seated properly on both cpu & gfx card and check what temp your motherboard resets the PC at as this might be set a touch too low.
Let us know how it goes