I just finished ( acouple weeks ago lol ) doing a very intense study of 11 most popular AV programs, using various well known software review sites plus testing them myself using
The results where quite surprising. At the bottom of the list where Norton's (especially on XP, it doesn't perform as well on XP for some reason ) and McAfees. The top AV's where AVG, Bitware Defender & Kaspersky, with PC-Cillin not far behind.
None of them though even gave 90% defence. Like the above mentioned site says, it is safer to use 2 AV's. Of which either of the 3 mixed gave 100% defence. I use AVG and System Suite 4's ( which is undoubtedly the best System Suite aroound now) built in AV which works very nicely. Bit Defender was the best all round AV program as it also does a firewall and has AV for ICQ, MSN & Yahoo but I found it to slow the computer down considerably.
Hope this helps