computer randomly reboots

my computer has been restarting randomly - like every five to fifteen minutes of usage. sometimes it even restarts while windows is starting up.

recently installed a video program and i thought that it might have changed the video settings that might have caused the problem. i did a system restore before the installation but my computer still reboots. it doesn't restart by itself in safe mode. when i unchecked the option to automatically restart when there's an error, it still reboots without showing any error.

my system:

windows xp home edition version 2002, sp2
pentium 4 cpu 3.00 ghz
3.01 ghz, 512mb ram

nvidia geforce fx5200