Two "active" antiviruses at the same time is a bad idea. A single active one and a one that only does manual scans, that's okay, but a bit wasteful of limited SSD
Which antivirus is best is largely a matter of opinion, different technicians have had good and bad experiences with most of them. Perhaps a better question is "What's an antivirus that isn't terrible"?
I've heard good things about Norton 2009 from different places. Many people like Kaspersky. Many swear by CA. If you like free ones, AVG and Avast are popular.
Personally, I prefer to not use Windows, but that may not work so well for you.
Vostro A90, 16GB SSD, 2GB DDR2, US-INTL Keyboard, FreeBSD 7.2-STABLE
Latitude E6400N, 2.4GHz, 4GB, 320GB, Opensolaris 2009.06 updated to build 118 Dell Tech Support agent