Microsoft Security Essentials – can’t update Virus & Spyware definitions.

Hello,  I have been using Security Essentials without problems until recently.  Now it keeps coming up with the message that Virus & Spyware definitions must be updated or my PC is at risk.  Whenever I have tried to update, it comes back with the message 'Virus & Spyware definitions couldn't be updated.  Check Internet or network connection and try again'.  There isn't a problem with my connection that I know of.  How do I solve this problem?  (My PC is using Windows Vista.)  Many thanks.