I have tried a few antivirus products before finding my current one such as these : Norman, E-trust, Panda, Trend Micro, McAfee, F-Secure, Inoculate, Dr-Web (free), Clam-Av(free), Microsoft SE (free), AVG (Full & Free), Bit Defender (Trial), Bull Dog (Trial) & Norton's Professional Edition just to name the ones that still come to mind..
All of these worked to a degree, but I still had mixed problems especially the computer slowing down drastically, at first I thought Norton's could not be beaten, but then I tried Trend Micro which left it for dead as far as slowing the computer down.
After all these I finally found Avast 4 Free, which saved me from a format after Norton's found nothing causing my computer to slow etc, Avast found what Norton's did not and I have been using it since and now use the full version.
I think anyone wanting an antivirus program they can trust should at least get Avast free or full version. I not saying it's perfect, no antivirus is, what I am saying is, it's the best choice available for now.