Ditto on the AVG.
Totally non-intrusive, very lightweight and highly effective.
Not to mention FREE!
http://free.avg.com/ww.download-avg-anti-virus-free-edition
I'm with you on the effectiveness, but not the other two adjectives.....ever since I installed the new version a couple of months ago, I've noticed that the daily scan takes three hours - and during this period my computer runs as if I'm on dialup again.