Be absolutely sure your computer doesn’t have any viruses Microsoft Security Essentials knows about by scanning your entire PC.
It’s a good idea every now and then to scan your entire computer for viruses just to make sure. Sometimes a virus will slip through Security Essentials’ real-time protection because it’s unknown by Security Essentials. Later, Security Essentials will receive an update and make that virus known. A full scan of your computer will make sure there are no known viruses hiding on your PC.