I've used most of the free, paid, consumer and enterprise versions of the anti-virus programs mentioned here and others as well. 3 years ago I took a job in IT at a school district who used Kaspersky. I spent MOST of my time the first year fighting viruses that Kaspersky did not prevent. Once I convinced the Systems Administrator to switch to MSE, the frequency of virus related issues has easily dropped by 75% or more, and consistently so over the past 2 years.
Depending on who is doing the evaluation, and many other factors, you'll see all sorts of anti-virus software comparisons with different results touted on the web. My personal experience tells me that MSE is at least "good enough," it's free, and not too obtrusive. Besides, you own personal computing habits and common sense are far more effective than any protection software. I do recommend Malwarebytes (also free) for its ability to detect and clean a PC if you do have a virus, although as bitflipper pointed out, much of what it finds isn't always deadly -- still a good effective tool that gets frequent updates.
On the rare occasion a nasty virus gets through MSE that isn't so easily removed, there's a point in time of fighting a virus that just isn't worth it anymore and the best option is to re-image the OS drive. I can't stress enough how important is to keep a full system image on another drive along with frequent backups of your user data. Windows 7/8 included backup and restore tools are capable, with more feature rich options available from Acronis, Paragon, and others.