These are the programs that I use to protect my PC against various threats such as viruses. There are other programs out there, but this combination works for me:
Norton Anti-Virus:
http://www.symantec.com/nav/nav_9xnt
ContentProtect, to guard against my kids seeing websites they shouldn't:
http://www.contentwatch.com/products/contentprotect.php
SpyBot Search & Destroy, to remove spyware and adware from the PC (free):
http://www.download.com/3000-8022-10122137.html
ZoneAlarm, stops hackers from hacking into your PC (free):
http://www.zonelabs.com/store/content/company/products/znalm/freeDownload.jsp