Hey Canadian. I don't know if you ever fixed your problem but I have the same kind of issues. But it's not MCfee related, i don't even use it, so I don't know if your problem was related to that either.
It seems to be a problem between the nature of p2p and VISTA configurations. I had the same problem with windows xp but not with the OS itself but with my router. It basically allowed all traffic until p2p in a way clotted the connection and webpages stopped opening. I had to get a script to change some settings on my router and it worked. I'm guessing the same kind of changes are needed on vista but I have no idea how to make them. I'm sure others have had similar problems. If anyone with the knowdlege could help out it would be appreciated.

Thanks.