Keep bad guys out of your computer system.

These days, any computer connected to the Internet needs to be secured. A personal firewall is a piece of software that is critical in securing your computer on the Internet. Personal firewalls monitor access attempts made to your computer from the Internet.

A good personal firewall is easy to install and use (so you can be sure you've set it up correctly and that it's doing its job), unobtrusive (so that it doesn't get in the way of your normal Internet activities), and inexpensive.

There are a number of personal firewall options available for Windows PCs and Macs.

Windows



BlackIce Defender and ZoneAlarm were two of the original Windows firewalls, and are still going strong. There's even a version of ZoneAlarm that's free for personal use. Norton Personal Firewall is also a mature product, and is the only personal firewall that also has a Macintosh version. All of these firewalls have recently been updated to support Windows XP.

Mac



Intego's NetBarrier was one of the first Mac personal firewalls. The Symantec product fully supports Mac OS X, and more firewalls are being developed for the new operating system every day. There are also a number of shareware options, such as BrickHouse, that work with the new OS.

Alan Oppenheimer is president and founder of Open Door Networks and co-author of "Internet Security for Your Macintosh: A Guide for the Rest of Us."