But I had the same problem with windows 7 and kaspersky a few weeks ago, I run some checks (downloaded the eicar virus test file) and kaspersky still picked it up, so I ignored the WSC message.
A couple of weeks later (following a windows update) the problem fixed itself...
So..... I'd suggest, firstly, doing a full windows update and see if that sorts it.
Microsoft Security Essentials updates with your automatic updates and this message is one of the pitfalls of this program . Like Chuck said make sure you update your computer regularly to prevent this message. This was one thing that was brougt to the attention of Microsoft after the program was released but as of late no action has been taken.This will happen also if you dont use your computer for a few says or longer. Just make sure you have your computer's automatic updates set to download at a time when you are using your computer.