Marti is having trouble updating virus definitions with Microsoft Security Essentials. Leo says it may be a stuck update. Once an update gets stuck, further updates wait until that stuck update is cleared before they install.
Leo advises simply uninstalling Microsoft Security Essentials and then reinstalling it. If that doesn't clear the issue, then Leo advises scanning with the Malicious Software Removal Tool (click start > run > type "MRT" > enter), and then choose "full scan". After that, Leo advises going to ESET and run their free online scanner.
If all that comes up clean, then Leo advises running Eset's Services Fixer.
(Disclaimer: ESET is a sponsor).