I think it depends a lot on your use of the computer what will work for you.
If you visit questionable websites, or download/run questionable software on your computer, then you probably should go with a commercial security package.
If you are the type who uses internet explorer and immediately clicks the 'protect my computer' link in any browser window that pops up saying "Your computer is infected with 123345234 threats!!!!", or freely clicks any attachments e-mailed to you, then a commercial package might be a good investment too (although no level of security, except possibly unplugging the network cable, can save you from determined stupidity
Otherwise, I think one of the free ones (possibly coupled with running an on-demand anti-malware scan (MBAM comes to mind) every now and then), is to a large extent more than sufficient.
Also, like superboyac notes, many of the commercial suites have gone from good to unmanageable giant piles of ... I used KAV/KIS since version 5, back then it was lean and powerful. Every version since then has gotten bigger, more bloated and slower. It is now at the point where you see special editions for laptops and notebooks, or recommendations to run older version on those, because the latest versions will bring any system that isn't quadcore with 4 gb+ to its knees
Also, I was getting sick of the way that buying the upgrade from within the software was more expensive than buying an upgrade box in a computer shop, and even that was often more expensive than buying a box of last years version in the shop and installing that and then simply upgrading to the latest version (*glaring at Symantec*).
I've tried some of the other free ones, but either they constantly nag you about upgrading to the paid version, or they have clunky upgrade procedures. MSE is free with no annoying adds or anything, and it updates automatically alongside your Windows