The best free antivirus is AVG. It's not truly great, but it's free, and it's the best of the free.
The best antivirus is NOD32 from Eset. It's truly great.
I took my laptop in to get a tune up and the guy turned me onto AVAST antivirus its free and really works great, I can't believe what it picks up, I had nortons 360 before and AVAST caught 6 viruses that 360 missed and to top it off it isn't hogging resources like 360 did.