If you're looking for a good antivirus software, try this one. http://free.grisoft.com/doc/1 Its good and it doesnt lag your system leaving it for fast response.
look in the tech section for a title called the 46 best app....has alll the free security tools u'll need in there..i'l bump it to the top...
A list of the best freeware security apps out there: http://www.dramasian.com/showthread.php?t=1898&highlight=apps