thanks for your help
CYaBro: Forget AVG and get the Microsoft Security Essentials.
I've found it to be a lot better than AVG and also Avast.
rvangelder: I keep Windows up to date, I don't do warez, I don't run document macros (unless I've written them).
So, I don't run a virus checker at all, and I've yet to be convinced that I need to.
Asmodeus: Is Microsoft Security Essentials unobtrusive? Has anyone got it (or any other similar product) installed on their HTPC? Mine is constantly connected to the net and I use it for surfing so wouldn't mind having something like this on it as long as it won't get in the way at all (Im about to install win 7 on it)