After using Avast and buying Kaspersky for use on Win 7 Pro 64.
I tried Microsoft Security Essentials and haven't looked back.
MSE is optimized for Windows 7 and performs flawlessly.
Loads fast and has a small footprint compared to others.
Full scans on my 1.5 tb are quick compared to the two aforementioned products.
You won't regret installing it on your system and best of all it's free!
Used in conjunction with Malwarebytes i've yet to have any issues.
I agree with Davcon. It's not the best, but it's very close. If you're looking for a free one, definitely get this one. And if you don't mind paying for good AV, well, you can save your money with this one, it really is as good as paid AV.
It was about time Microsoft offered their own protection for free.