PeerGuardian 2 is an IP blocker for Windows. It integrates support for multiple lists, list editing, automatic updates, and blocking all of IPv4 (TCP, UDP, ICMP, etc), making it the safest and easiest way to protect your privacy on P2P networks.
The first version I used was a CPU hog and required intricate and manual driver installation to work properly. The hassle is now gone and version 2 is easy enough for anyone to install.
Nordquist Blog 