UofT provides free licenses to Symantec Anti-Virus (SAV) and the newer Symantec Endpoint Protection (SEP) for staff, students and faculty for use on- and off-campus.
SEP is like SAV but adds a built-in anti-spyware program. For details and downloads, go to:
Recently available is Microsoft's Security Essentials, which is similar to SEP but is possibly superior. Under license only for home use. For details and downloads, go to:
http://www.microsoft.com/Security_Essentials/
Finally, there are those who swear by AVG anti-virus and anti-spyware software. Others find its default settings slow down their computers. The basic and sufficient version is free and available at:
http://free.avg.com/ca-en/homepage
Make sure you only have one of the above progams installed at a time - they can possibly clash with each other.