I personally used Symantec Corp in the past however its huge overhead and tendency to crash free korean games (like gunbound) was the reason I ditched it. Ive heard from some in the corporate enviornment that TrendMicro produces the best anti-virus software (largest database and great detection) however Avast! is a great alternative to those who need a free virus scanner. Im using AVG on my laptop now and am thoroughly satisfied as it instantly stopped a virus from executing from a webpage and cleaned it from the temporary files, however i had AVG-Antispyware installed also (uses the old ewido engine) which helped.
My recommendations have to be Avast! and AVG Anti-spyware of you are looking for a free virus scanner however
Ive used a bunch in the past, from avg free edition to bitdefender full. I found that avg free did a great job. Bitdefender did even better. But bitdefender tied up to much memory on my terrible computer and i find it runs much faster using the trial of windows live onecare. But ill have to switch back to avg when its over.