Microsoft Security Essentials is no longer good for antivirus...here's another option.....

Here's the best option on my recommendation at this time for 2013:

Antivirus Free Edition

A free antivirus that uses #1 ranked technology and is feather-light.

FREE DOWNLOAD

 

They only ask that you create a login and it is spyware free (obviously) and then it runs virtually unknown to you and keeps you safe!