Microsoft Security Essentials

I've tested MSE on several computers for over two months - is seems to be a bit lighter on resources and interface is decent (why does every antivirus MUST have some custom interface with ludicrous colors and interface styling?). On the other hand, to complete removing a virus from USB flash drive - you have to reboot (this is not very convenient). And finally, one of the computers caught some bogus fake antivirus malware, which somehow bypassed MSE and removed it from startup.

Real-time protection seemed to hog a lot of CPU for me (up to 20-25% on a Q6600) while scanning. RAM usage was also high (70-80 MB).

By far the best free anti-malware program available. It's easy to use, a single interface, virus signatures updated daily and very light weight on system resources. I cannot recommend this highly enough. Better than AVG, Avast and Avira.
Also, this software wasn't made by Microsoft. It was made by some security software companies that Microsoft acquired.

Lightweight, ad-free, free, and easy to manage oh, and it has rootkit protection. Frankly, Security Essentials beats AVG and Avira in my opinion, despite it being created by Microsoft (possibly the only drawback).

Fast, free, and gets out of your way. It's the polar opposite of Norton/Symantec. Wonderful app.

Seems to be a decent light on the resources Anti Virus. If you want a whole suite of tools, this isn't for you. If you need something to put on your Grandparents computer who don't understand Anti Virus software and don't want to spend the money, then it's perfect.