AVG Antivirus is free and it doesn't use up much resources like Norton's paid option.
I recommend getting Grisoft's AVG Antivirus Free Edition, I use it all the time, the scanning is good and it detects viruses as they come. (Like if you download a program and you open it and there's a virus) if you keep it running in the background. The updates are pretty fast too and easy to use.
Otherwise, get Norton.