so me and a few friends put together a puter from some old parts we had layin around and it will randomly restart itself. For the most part it seems to do it when you have a load on it such as a game, watching a movie, or sometimes when you try to transfer a large file from HDD - Flash drive. We started thinking the GPU was overheating so we threw a fan on it... no luck, so we then thought the CPU might have been overheating from bad thermal paste so we got some new and put it on... again no luck, so we then looked into power thinking it wasn't getting enough so we threw in a 420W PSU.... yet again no luck. We are completely out of idea on whats going wrong the last thing I can think of is the GPU is bad, if it was would it cause it to randomly reboot?, any help would be great. Its got AMD Opteron 2.6GHz Socket 939 CPU, 2 one GB sticks of RAM, ATI X300 GPU and its running XP.