Or a virus. There was one out a while back that caused an endless restart cycle.

I have a similar problem with my laptop.It rebots a couple of times when you irst boot it up but once it has done this a couple of times it works perfectly.
Anyone have any ideas I currently have the secondary ram card out as I was told this was the cause.
Thanks and sorry for the thread hijack Matty

Pretty much any removable part on a PC can become 'temperamental' where it connects to the motherboard. My last 'rebooting' PC was a memory stick that needed it's contacts cleaned up (matty, this sounds like what you have?).

First thing I'd do is take everything apart, clean it up and put it together again. The action of unplugging everything is usually enough to clean the connections enough. And while you're there you can check the CPU heatsink and replace the copper paste.

If it's overheating it would switch off, not reboot

If its an automatic restart cause by software aka a blue screen they you can turn off the auto restart if its Windows.

In XP and 7 right click on the computer icon on start menu and select properties.

If windows 7 click on advanded system settings on left (Not required on XP)

Then click on the Advanced tab, under startup and recovery click on settings

Under system failure there will be an option for automatic restart, make sure the tick is removed.

If it now blue screens it wont restart itself it will just show the blue screen with the error.

If its hardware then do as others have said, clean out dust from heatsinks, remove and reseat all removeable stuff such as memory, graphics cards, etc.

If it's none of the above, then another more random/extreme possibility that I have come across a few times and could be a candidate if it is an older PC are failing capacitors on the Motherboard, you can tell by the tops being domed. This was a common fault on a series of Dells for a while.