i use avg, mostly because decent rating and freebie, but it is not the best rated by any means. kaspersky is generally highly rated. do a search on "antivirus review" first to get better info than anonymous replies on ET.
very important is to download and run SpywareBlaster.com (then update it every 2 weeks or so). It prevents many things from even getting
Also, a firewall. The one built into Windows, ZoneAlarm (if XP), etc. etc.