Microsoft Security Essentials
A newer version of Microsoft Security Essentials is available.Or, Learn More About Microsoft Security Essentials
Full user review
"Great software for free"
Simple: it is free, it isn't harsh on your computer's RAM, it can set restore points (optional feature), quarantine feature, history, no need to worry too much about definitions as Windows Update can handle the definition updates (you can manually check for them as well), you can also choose whether you want the program to check for definition updates before running a scheduled scan
not as powerful as a few select other competing software
There might have been some things I missed, but really, you get a free security software from a well-known, trusted corporation.
I've been a user of this software for a long while, I have it installed on my Windows 7 desktop computer and Windows XP laptop, and there is a lot that I love about the software.
while it is true that you don't get protection that is as powerful as some other security software, you can't complain when you have something free and works great