May be a low-end "fix". Had this prob with client PC at work. Could not figure it out so was changing out to another exact same make/model (Dell Opti 745). New machine booted fine and everything, but AFTER ADDING ON ALL THE USB STUFF the user had plugged in, THE NEW PC DID EXACTLY THE SAME THING.
So, I only left the keyboard, mouse, and all-in-one ptr/fax/scn plugged in, and all was good.
I think it may be an underpowered supply.