PeerBlock lets you control who your computer "talks to" on the Internet. By selecting appropriate lists of "known bad" computers, you can block communication with advertising or spyware oriented servers, computers monitoring your p2p activities, computers which have been "hacked", even entire countries!
They can't get in to your computer, and your computer won't try to send them anything either.
And best of all, it's free!
PeerBlock is a continuation of PeerGuardian.