I highly recommend Microsoft's Security Essentials for PCs. Paid anti-virus apps do more, but unless you're constantly engaging in reckless behavior online, MSE should be perfectly adequate. Plus I've found that it's a light user of system resources and it doesn't constantly barrage you with unnecessary warning pop-ups.
I've decided to rely on the free Microsoft ("MS") Essential Security Suite, after bad experiences with two other products provided by third parties. When I talk to my more sophisticated buddies they sneer at this MS free stuff, and infer that if I knew anything at all I'd be on the non-free (or sometimes free but non-MS) system they use. When I press them for reasons on why their choice is better, the only consistent theme is that they really, really don't like MS, to the point where they'll buy an inferior product rather than let Gates "win" again by using his stuff when there is an alternative. I acknowledge that MS has dropped features from Essentials that formerly were in MS's "One Care" suite, no longer available. However, there are free ways to get things like automated backup and management of several computers for the casual user, and industrial-strength products available at commercial rates.
If your needs are covered by MS Essentials is there any non-emotional reason to pick another system? I will acknowledge that some of those systems have features that Essentials doesn't, but for me they are more featureitis than a pressing need of mine. Are all the third-party suites fated to disappear, like Netscape, or is the market broad enough for both?
And then there's the topic of why doesn't MS market Essentials more aggressively, but I suspect that question belongs on a blog on legal issues rather than this one.