If you've been running Microsoft Security Essentials as your antivirus, you should absolutely replace it. MS has so much as said they plan their AV to be a bare minimum and not much else. When it was new, it was testing very well but has been slipping consistently.
I used to be an advocate of free AVs but, after a plague of infections two years ago where most of the infected PCs were using free AVs, have been forced to reconsider.
As a result, I have found "Managed Antivirus" (MAV) which I can not only provide to users but help strengthen as well. When I clean up a PC and remove tons of toolbar/browser hijacks/etc, I save the scanner's log file and can add much of the files to MAV's blacklist. In this way, I can keep a lot of non-viral junk off of my customer's systems.
If you would like more protection on your system, contact Prompt Care.