Since your friend seems to not have a clue, simply tell him:
What I did, was the equivalent of a surgeon removing a cancer that had spread all over the body. The difference is, in removing the cancer, apparently I damaged one of the organs. In the case of a surgeon, you're screwed. But, in the case of the computer, it would have been very simple for me to replace that organ. And, it's only because you refused to let me take the computer home over-night that I lacked the ability to realize that the organ was damaged. Then, give him a list of 4 or 5 things that it could have been (I tend to agree with the TCP/IP being screwed up; I seem to recall having a similar problem) and tell your friend that "Wow, I can't believe you paid $300... you got ripped off! Even completely erasing your hard drive and re-installing windows from scratch would have taken only 30 minutes; backing up important files another 30 minutes or so. You really paid someone $300 an hour for repairs? Or, didn't that person know what they were doing?
That said, I went through a very similar thing with the in-laws. It *NEEDED* to be reformatted. But, I wasn't allowed to do that. It took me 2 days to fix it, removing thousands of viruses, spyware programs, etc. I spent 2 hours at their house before I insisted, "this HAS to go to my house to be fixed. It would take weeks to fix it without my own computer to work on it with." In the end, I installed a free anti-virus program on the computer and set it to auto-update, auto-scan, and to automatically quarantine any viruses it found. 3 months later, "That piece of crap anti virus program you put on my computer doesn't work. My son had me install another anti-virus program and it found 3 viruses." Me: "Really??! Let me see." "Here... I found where those viruses are on your computer. For your information, MY anti-virus program found 8 viruses and quarantined them all. Your program found 3 of the quarantined viruses. So, it seems that it's YOUR program that sucks. And, what are you doing that you have 8 viruses?! I've only ever had 1 virus, and it's because the kids downloaded something when they were little. Furthermore, the reason your computer has ****** the bed again is because you refused to listen to me and are installing all these things that I warned you against. I specifically said NOT to install comet cursor any more."