Well, as you said, I would recommand formatting the hard drive, his computer would look like it's new. I often had problems like this with old computers, and a computer that wasn't formatted for 8 years!!! (hehe, I'm a formatting freak, as soon as something does not work, that's what I do..) Especially from an old guy, when he surfed over the internet, he must have said yes to every pop-up and said "next" to everything he installed without checking what it said because he didn't understand anyway. So his computer must be filled with half-removed programs, spywares, as New_Leaf said and all kind of crap, maybe even viruses.
Try asking the guy, maybe he has no important stuff to save, and if he has, you can even back all his stuff with just a serial or parallel port and cable and transfer his stuff to another computer.
Hope this helps a bit