Just following up on your comment - why would you not run a firewall as well as an AV product on a windows box? Firewalls should be low enough load not to impact your CPU adversely in any significant way, and anyone on a broadband link is a target for any number of scans followed up by attacks. My advice, as a security professional for the last 15 years, is just to get the basics in there before the decision comes back to bite you...