I always used to work with AVG, but then one day my PC started freezing up for no reason. It took me to reformat the computer, re-install windows and noticing the freezing problem re-occur as soon as AVG was installed. Don't know if they've fixed the issue since then, but I've switched to Avira since then and it works great. I've used MSE as well, but it does miss viruses more often than the other two in my experience.
I recently decided to actually buy a proper anti-virus, and after some researching and comparing the detection rates, false positives and prices, I decided to just go with the Avira Premium which seems like the best value for money and so far I've been very happy.
I found an email a while ago containing a new virus (Fedex email containing zip file... back when those emails just started) and Avira did not pick it up so I simply forwarded to them. That night my virus definitions were updated and the next day when I received that same email again, it was blocked.
I'd say their free version is probably the best of the free versions around and their premium edition, although not the best, it is the cheapest and does a pretty much the same job of anti virus products twice the price. Just google "Compare anti virus" and choose one of the first two resuls after the ads... there are some nice pages where they compare specs, features and reviews in tables with all the products side-by-side.