I'm using Avira antivirus right now, it does a good job and its free. Don't think you can really beat it, myself.

But I'll echo others; DON"T use Norton. Not only does it cost cash, but it takes forever to scan, and to top it off, Norton is the base anti-virus the majority of skilled virus makers test against(this coming from my hacker friend, who of all people would know), so Norton is actually more susceptible than most of the free anti-viruses.