computer keeps rebooting

Ok here the problem. Computer keeps rebooting itself. Computer starts up fine and loads up all the desktop items. When I go into any of the programs the computer shuts down and reboots itself. A few things I have already tried are looking at the cooling fans. Both are working. I have changed the hard drive from my other computer but the same problem. Reloaded windows xp still same problem. Anyone have this problem before? I'm hoping its not the motherboard.
Thanks

 

Can you boot the pc in safe mode? If so, try to open a program from there and see what happens. sounds like a possible lack of system resources. Also, while you're there, you might want to run a virus and spyware scan.

Hope that helps..

 

Tried running from safe mode and still having the same problem. I cleaned off the hard drive and re-installed windows. Computer is still rebooting.