Microsoft Security Essentials is now free, and works great. You can install it after disabling your other AV program, just do not keep more than one AV program on at a time. The latest internet scourge seems to be trojans that the Microsoft Security Essentials was able to get off my system, after others did not.

Mininova is a bittorrent/ hacker site where you get illegal downloads for "free" - you also get lots of trojans and viruses attached to those freebies, so be sure to run a virus scan on anything that comes off that site.