I'm had been using AVAST too, for 1 year and more already. It's great!
It's installed as Window Service so it starts everytime your computer is on.
It provides on access scan, as well as scanning your outlook emails, P2P files, Web Pages and more. And most important is its virus defination database is keep updating automatically whenever there's new patch!
Great for home use as it's FREE. For commercial use you'll need a license. But it's function is as good as all famous virus scanners.