I vote double what everybody said about Norton - I've never liked its performance ding. A former job used McAfee: it was effective, but a little slow IMHO.
I'd suggest AVG Free. I've (re-)built 100's of systems for friends and family and always put it on first thing after taking off whatever they had previously. It's on all 4 of my home systems, and I (gasp) actually started paying them for it a couple of years ago.
Another 100% A+++ product is Sophos. The last time I used it was in a commercial setting where it wasn't my money! It still is quasi-commercial only -- have to buy >=3 licenses. The AV-only product is about $45/seat for 1 year, $136/seat for 5 years. The AV+firewall is $59/$177 respectively.
I've not had any experience with MSE or NOD, but haven't heard anything negative about either.
I also echo the "only run one antivirus at a time" advice. At best, one may false-trigger the other and at worst they can Blue Screen.
Lastly, I came across
http://forums.majorgeeks.com/showthread.php?t=35407 years ago. True, it is a guide for cleaning a 'dirty' box, but it points to a lot of useful info and tools that can be just as effective on a (supposedly) clean one as well. The writing is a little convoluted, and the interface for finding other stuff on their site is clunky, but I've always had good results following their lead.
Happy hunting, and congrats on being rich!!
