"It will boot then restart". How far does it get, all the way to the Desktop?

You haven't given any information about the computer, other than it is an HP. Details would be nice as asked for in red above each posting window.

Can you boot up into the Safe Mode list of options? Instructions here; http://www.bleepingcomputer.com/tutorials/tutorial61.html#winxo
If so, what happens if you choose the first option, start in Safe Mode.

Assuming this is Windows, what happens when you insert the Windows CD when you boot up?