Best anti virus out there is:
http://www.avast.com/
It is free for home use. I have been using it for last 2 years without any problem. Updates itself automatically and never failed on me. I would rate it 100 times better than norton or any other anti-virus.
You can download the home version:
http://www.avast.com/eng/down_home.html
Just
register with them free and you will get your registration key in email free.