There's no need to purchase antivirus software. I have not paid for one single software package, anitivirus or other. There's plenty of free applications available, and they're just as good or better than the commercial packages.

Check out this website for a huge list of free software (known as freeware). It details each program and lists its pro's and con's.
http://www.techsupportalert.com/

And here's the link to their security section, containing antivirus, malware and firewall programs:
http://www.techsupportalert.com/pc/security-tools.html