Windows XP comes with a built-in firewall which is better than nothing but just barely. There are a lot of 3rd party firewalls that do a lot better job and some of them are free. Most all virii have a hidden 3rd file extension i.e. .exe, .pif, .scr, among others. I have only used Zone Alarm so I can't speak to the others but ZA will quarantine any programs with a hidden file extension so you can decide if it is safe to open it. If anyone uses something else with the same feature, post it in software. If you do install a firewall, here is a great place to test it and see if it is working. http://grc.com/x/ne.dll?rh1dkyd2 Also try this test. http://grc.com/lt/howtouse.htm A properly configured firewall will show you "invisible". Look for a class on firewalls in 101.