For free it's difficult to recommend anything other than Microsoft Security Essentials. It just does the job, fuss free. No added fat as is the case with most free AV products.
As for commercial products, I'd only recommend NOD32. Kaspersky isn't quite what it was (it was up there with NOD32) but it's still a very good antivirus product, and it's free if you do online banking with Barclays.
I can vouch for Microsoft Security Essentials. I installed it on my laptop after it got upgraded in early 2010, and so far it's been epic, no issues at all. Actually Gav, I found it by remembering a link to it which you posted here a couple of years back, so I owe you major thanks on that.
I too run MSE now. I just couldn't justify the price of NOD32 now that a decent free alternative is out. I'd still have a NOD32 subscription if I was limited to Avast or AVG for free products.
Apparently Norton's version for last year was a great improvement, but then it was hard to make the product worse. It's still only there for non-computer literate.