Free: Avira Antivir, Grisoft AVG, Avast!
Paid: Kaspersky, NOD32
Personally, I use Avira on all my systems now. BTW, I strongly recommend looking at the test results from http://www.av-comparatives.org/
and seeing what various scanners can do against a variety of virus/trojan types. As others have stated, stay the hell away from Norton/McAfee unless you like your system resources being gobbled up for no reason.