Check the PC date and time – right click the clock in the tray and select adjust date and
Verify and correct the month, day, year, and time. Reboot.
If this does not resolve it, contact our support agent at : http://www.microsoftliveassist.com/
You can manually download the (large file with engine and full definitions) latest definitions from the portal here: https://www.microsoft.com/security/portal/default.aspx