Dominique is right – there is no one thing.
I’m using Trend Micro Security.
ZoneAlarm is great, also.
On top of that I have Ad-Watch running constantly.
I could have more, but in all this time what I have has been working fine.
You can scan your computer for viruses online here: http://housecall.trendmicro.com/housecall/start_corp.asp