Another one for the new people

Who are just starting out. My computer runs great at first but it reboots by itself after a few minutes. Is it the power supply? I have a HP vectra vl series 5/75 and it ran fine last week. No new programs or hardware. I checked the memory sticks and they seem to be ok. I have 64 mb on a win95 computer. We are on a NT network with exchange server.