Hi Dead comp,
You could try to stop the cycle by stopping the reboot on system failure. You dont mention the version of Windows, but i'll guess its 7 or before ... To do the job you need hit the [F8] key repeatedly after you switch on. Contilue to do this until the Advanced Boot Menu appears. With the arrow keys select Disable Automatic Restart On System Failure and hit [ENTER]. Hopefully you will get a blue screen with white writing on with a stop code. Let me know the code and I will try to help from there.
Also if we cant get it going again, there might be a way of retrieving your data, but it depends on the damage caused so far.
Good luck - dont forget to post back with the stop code.