the one i use and it is no cost to you is avast antivirus. it is free for home users and has been raited better than norton. avast on google or avg. well let me give you the sites, http://avast.com/
and avg is http://free.grisoft.com/doc/2/lng/us/tpl/v5
for avg you do not have to register but for avast you do. they give a one year registration code. i have both and i like it alot. a computer tech gave the information. avast i like a bit better, you can actually scan you computer on start up. it works great, try it. i have so many viruses on my computer for downloading, so i know, how to feel bad. the good thing about avast you can put the files into a chest instead of deleting them, and it also ask you if you want to repair the file so you could put it back or use it again.