NOD32 by Eset. Installed it just over a year ago now and it's utterly superb. All the things you need: auto updates (sometimes as many as 6 per day!), full seamless email/IM scanning, quarantines, un-beaten track record for actually catching all viruses and it's FAST. Doesn't hog masses of CPU or memory, you don't even know it's there (something that you can't say about Norton).
I will never use a Symantec product ever again having moved from 6 years of using Norton AV to NOD32 - it felt like my CPU got upgraded for free
Cheers,
Rich
Two Worlds and in Between
Hot Metal and Methedrine