Free a/v for Windows?

Linda’s mother was asking about antivirus software for her Windows machine after I cleaned her machine up from spyware. She had NAV 2005 that “came with” her PC, but it was one of those trial thingies and the trial expired. I told her I’d look, and honestly, basically forgot about it because, well, it’s not my PC.
I saw this the other day as it went through my RSS reader, and figured I’d have a look. Kind of a lightweight review, but it’s a start. I may see if ClamAV has been ported to WIndows as well – I know it had been to the Mac.
(My huge bookmarks file contains mostly bookmarks of things like this that I’d note and then forget about, and it’s annoying me. I’ve been culling the bookmarks, and figured I’d adopt a more Richard Bejtlich approach: no bookmarks, just a weblog posting and a note about why the thing was of any note at all. I’ll see how it works out. I expect that will make me a fair bit more chatty.)