Because malicious software is continually being developed, Microsoft Security Essentials relies on up-to-date definitions to determine if software that is trying to install, run, or change settings on your computer is potentially unwanted or malicious. Microsoft Security Essentials updates the virus and spyware definitions on your computer automatically. If the definitions haven’t been updated for over seven days (for example, if you didn’t turn on your computer for a week), Microsoft Security Essentials will notify you that the definitions are out of date.