Norton 360 was the "Editor's Choice" suite per PC Magazine this year and got especially high marks for the low impact it had on performance. Here's their review:
The Best Back-to-School Software - Norton 360 Version 3.0 - Full Review - Reviews by PC Magazine
I could run the free McAfee anti-virus that I get from Comcast, but the Norton has many more features and I could get a 3-box license for 65 bucks at Staples.