I use NAV, it works well and it doesn't get in my way. I used to use something called Thunderbyte, but they were bought by another company who ruined the program.
One thing you have to remember about AV software, they are not foolproof, they may not all always catch everything, and you have to keep them updated for them to properly protect. Also, heuristic scan is a good thing, it can give false alarms but it rarely lets anything slip through.