This software protects you against the modern plague, viruses, trojans or worms whatever they're referred as. I think most of the infections are from email attachments. Although I'm not going to preach about that here, but I'll just say don't open everything, that 5 meters long gorgeous penis or a eternal happy life is just not possible (for everybody =)). Think before you act! Use a email reader that has a SPAM filter, personally I use Qualcomm's Eudora but there is also free choices available such as Mozilla's Thunderbird. A good anti-virus software guards you by monitoring what files you're trying to execute by quickly scanning them before you start them. And updates itself daily with an up-to-date virus database to catch the newest threats.
F-Secure Anti-Virus, F-Secure
Absolutely the best! I've always used this one. Although not free, but definitely worth the money. They have this new Internet Security 2005 with firewall and the anti-virus in a low cost package. I've considered starting to use the package.
AntiVir, H+BEDV Datentechnik GmbH, free!
Not as easy to use. Updating needs little personal attention, but instructed and notified when needed. Little complicated at first but delivers what it promises!