PeerGuardian is a pretty effective IP blocker for BitTorrent and other file sharing applications, which is very simple to use. They have a Release Candidate test of version 2.0 available at the moment, which claims to support Windows Vista, unlike their current stable version. This can be downloaded from Softpedia here.
If you have already tried using this test version or prefer not to use test software, one thing that would be worth trying is creating a BartPE boot disc with a portable Firefox version(to browse for Torrents) and run your Torrent software and PeerGuardian on this. I haven’t tried this so far, so am not sure if PeerGuardian will run in a PE environment. As BartPE does not seem to include network drivers that work with Virual PC, the PC would need to directly boot from the disc, although this may not be much of an issue if you mainly leave your PC idle with Torrents running.