PeerBlock is similar to Peerguardian, a premier IP blocker for Windows, but includes bug fixes for various issues found in Peerguardian when it comes to Vista and Windows 7.
PeerBlock efficiently safeguard your privacy, whether you are sharing files with Bittorrent or just surfing the web, it won’t allow ‘others’ (block listed IPs) to access your PC.
Installation and usage of this program is pretty simple (details here). You can enable or disable blocking feature from the system tray. The automatic update of block lists ensures more security.