My main job is cleaning virus infections off of students' computers, so I deal with this a lot. I used to use Avast a whole lot, and recommended it to a lot of people, but in the past few years it's seemed to become a lot more bloated and over complicated than necessary. Likewise with AVG, which I feel has gone even further downhill in a similar way. On a Windows machine, I think you'd be very well served with Microsoft Security Essentials. It may be free, and it may be from Microsoft, but I've found it to be excellent. It's a very lightweight program that takes up comparatively few system resources, it scans quickly, and it finds many things that others struggle with. The rest of my IT department colleagues feel the same way, enough so that we've recently switched our entire company contract to the corporate version of it (Forefront).
The best recommendation I know for MSE? All the computer geeks I know put it on their parents' computers.