[Justa]

I use Microsoft Security Essentials and Malware Bytes. Before MSSE I used Avast!. It was much better than Symantec or McCaffe.

[Hardraade]

Avast is very good. So is malwarebytes for the rootkits and stuff.

All others, avoid like the plague. They are bloats, or even worse: hacker hangouts, malware authors or jihad pushers not to be trusted at any level.

[Yosemitest]

I tried Microsoft Security Essentials and found it to be a CPU hog, and not very good.
I dumped it.
Take a look at Advanced System Optimizer 3 and check out its standing and reviews.