PC Tools Firewall Plus is a personal free firewall for Windows 2000, XP, 2003 Server and Vista. PC Tools Firewall Plus provides protection from unauthorized access, hackers, key loggers, Trojans, worms and much more. PC Tools Firewall Plus is a completely free software and is designed in a way that both novices and experts can use it to their levels of usages. However it can not function on its own to provide complete security, there needs to be an anti virus and an anti spyware  installed alongside for full and broad protection.
The Version 4 has improved leak-test protection, which means that your system is safeguarded by this software without any unguarded spaces in the program that may allow intruders to pass in. Better Leak-Protection lead to better firewall ultimately leading to a completely protected PC.