From the publisher: 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 Portable includes the 32-bit and 64-bit variants of both the legacy and modern versions of PeerBlock, automatically selecting the appropriate one, for full support on every PC you use.