|That plug gives some extra current to the cpu. Once or twice I've seen where disconnecting it causes the fans to spin up. I think it's a motherboard problem.|
As I mentioned in # 2 above there's no practical way for a user to test the parts except for swapping them out, unless you take it to a shop.
Well I remember seeing some add-on cards on ebay that supposedly can diagnose a dead system. I've never tried them though.
Take a close look at the capacitors that (usually) are lined up alongside the cpu. They'll be little cylinders standing on end. Do the tops look a little puffed out like there's too much pressure inside? When they get that way the motherboard is on its way out.
Real men don't use AntiVirus; they just reformat