Kaspersky and Norton are solid choices, Kaspersky has an option that only scans new or modified files and Norton just performs well. If you have a router and aren't opening alot of ports the windows firewall is fine.
Another option would be Prevx, very low resources and better than an anti-virus for realtime protection.
Prevx review: http://www.youtube.com/watch?v=AAx6Y...e=channel_page
Norton review: http://www.youtube.com/watch?v=yU9U4...e=channel_page