Is it a continuous reboot loop? Or does it cut off and stop until you physically turn it on again.
Is this the first time it's been boot up? No OS is currently on the system correct? I would double-check and make sure everything is plugged where it's supposed to be inside the case to your motherboard.
If there's no OS on the system and this is happeneing for the first time upon startup, something isn't right. Give the system specs so we can make sure all the parts are compatible. Perhaps the PSU can't supply enough power to boot...or something else along those lines.
Where exactly is it on the screen when it restarts itself? BIOS loading up and detecting IDE devices? Or before that?
I'll help more if I can when I know the system specs. Thanks.
AMD 3400+ 2.4GHz
1GB DDR Crucial RAM