I'm not really a Windows person, but on the few MS machines we have we use Avast free edition. With the popup/notification option turned off it's quiet and doesn't get in the way (apart from the yearly nag to upgrade or re-register the free version). It's not caused any problems, but did correctly identify a virus on my other half's PC a few months ago.
Why does Avast have such a bad reputation? Most of the other options I've seen have a massive negative effect on performance.
If it wasn't for Cubase (and the cost of a mac) I'd be 100% Linux
