i had that problem once before, it was even rebooting for
the BIOS. have you added any cards, fans, lights, memory,
drives or upgraded the processor? if you have there is a
chance that you are having trouble with you power supply.
power supplies included in OEM computers tend to be
really bad, also the ones that come with cases are very
bad also. if it will run for a few minutes then start
rebooting like crazy you may be having a heat issue,
check to see that your power supply can is turning and
that the air flow everywhere on your case is not
obstructed. let me know if that helps.

Carl
Ciffer25@aol.com