It's possible that the computer has truley had every last gap in windows filled with spyware. Or, possibly a fault with his router or firewall?
Recently I had to reformat a computer due to internet not working properly, I tried re-installing TCP/IP and all sorts. Make sure he hasn't overloaded his computer with internet security... Norton has always been evil for me - never really turns off, just pretends to. Stick with 'lightweight' firewalls, or a hardware firewall. And, despite all the arguments, FireFox is definately the safest browser to use - they release patches much faster than Microsoft do for their Internet Explorer. Plus, the amount of extensions available helps alot (Adblock and the likes).
Sometimes the only option is to reformat, as by the time you've figured out what's up, you could have reformatted and got a nice clean computer up and running..!