My mother had some e-mail virii that Norton and Symantec quaranteened but could not fix or remove. She updated the definitions over many many months and they still could never do anything (they just knew it was bad).
I told her what I was using, she removed the others and put AVG Free on, and not only did it fix/delete the existing viruses instantly, it also found some others that the "name brand" software couldn't even see.
AVG Free for me, from now on!