I can recommend 2 free AntiVirus progs which don't eat your PC's performance & resources. you must tweak them though.
Avira AntiVir
[
www.free-av.com]
Avast
[
www.avast.com]
btw, both progs are better rated in performance and detection than Symantec.
symantecs Norton used to be good in windows 98 days.