When our McAfee subscription recently expired I decided to switch back to a free virus scanner. AVG was always the most commonly-used one and I installed it only to find that the latest versions aren't as good as the previous ones. The newest version was a memory hog and caused my computer to run much slower. So, I ditched it and set off to find a better one.
Avira is the only free one I've found to be fast and reliable. It automatically updates upon boot-up and then displays a pop up advertisement. That's a very small hassle for this great product. It's a British company so the prices are usually in pound sterling.
Avira found stuff on my computer that neither McAfee nor AVG found. I highly recommend it. You can download it here.