The most popular free anti-virus program, with the 'regulars' here in AB's Technology section, is AVG Free Edition:
http://free.avg.com/download-avg-anti-virus-fr ee-edition
However, some people have reported dissatisfaction with the latest version, resulting in them switching to Avast! Home Edition:
http://www.avast.com/eng/download-avast-home.h tml
You'll also need a decent firewall. Zone Alarm Free is frequently recommended here on AB:
http://www.zonealarm.com/store/content/company /products/znalm/freeDownload.jsp
(NB: Use the right-hand blue link. The more prominent green and yellow link downloads a time-limited trial version of the full commercial suite).
Some users prefer Comodo:
http://www.personalfirewall.comodo.com/downloa d_firewall.html
You should never have more than one anti-virus program and one firewall running on your PC (otherwise serious system conflicts may result). However, you can have several anti-spyware programs (which run 'on demand' rather than 'in the background') on your PC. Many people have both Adaware Free and SpyBot Search & Destroy:
http://www.lavasoft.com/products/ad_aware_free .php
http://www.safer-networking.org/en/mirrors/ind ex.html
You should definitely get rid of Norton. (It's notorious