I cleaned up a PC that had over 500 (was several years ago, don't remember the exact number) infections. They were a friend of my dad's so I just charged him some gas money, since all I did was throw in an unattended XP CD and waited for it to finish.
I received a phone call from him before I had even made it home... about 15 min away. The AV I installed had already found several viruses on there. He got mad and said he was taking it to Best Buy. After they charged him $350 to reinstall Windows like I had just done, he was more than willing to sit down with me and let me show him how to be more secure with his computer use.