The antivirus software below is free. Free antivirus software may be the solution to your problem, but often times paid software does a better job. In my opinion this is one of those times.
Check out NOD32 in My Antivirus Picks by clicking the link at the left.
Avira AntiVir - Very good virus protection with Virus Guard real time monitoring. Employs a heuristic (behavior based) system in addition to traditional definition based protection, which helps guard against both known and unknown threats. Protects against all types of malware including rootkits and includes the ability to schedule scans. Currently rated in the top 5 Antivirus products for detection, but suffers a little from inconsistent updating and lack of email scanner. (XP/Vista 32/64bit)
Avast Home Edition - Offers an extensive array of protective shields including; instant messaging, internet mail, network, P2P, web and standard (general). Each shield’s sensitivity level can be set individually. Also includes an option to run a boot-time scan and very good virus cleanup tools. Comprehensive protection from all types of malware, including rootkits. A full featured, set and forget solution. Although free it does require annual registration. (XP/Vista 32/64bit)
AVG Free - Popular, solid anti virus, however the latest free version (8) seems to have taken some backward steps. There is no protection against rootkits and many users report computer slow downs due to AVG’s heavy use of resources. There have been complaints too about the new ‘Linkscanner’ feature which adds a toolbar to browsers by default, many are unhappy that it is compulsory. (XP/Vista 32/64bit)
Threatfire - A standout in the free Host Intrusion Protection System (HIPS) category, Threatfire is unobtrusive, fully automatic and provides a vital extra layer of protection. Terrific adjunct to existing traditional anti-virus. (XP/Vista 32/64bit)