Microsoft Security Essentials is my favourite so far: it's light and minimalistic, I love it.
However if you're torn between Avast and AVG I'd go with Avast. I used to go with AVG but as already mentioned it did start to get bloated. The extra process running in the background is just one excess, there's also the occasional pop-up that advertises the "pro" or "full" edition.
Avast have made some major strides in making their free edition a nice and tidy little package.
I have also used
Avira Antivir Personal, which is also pretty good and free but I did notice a "pro" advert pop-up at one point.
Alternatively, if you're a careful user anyway and don't care for a resident shield but would like a reliable virus scanner you could go with
ClamWin, it's opensource.