Definitions are files that act like an encyclopedia of known spyware and other potentially unwanted software. Because spyware is continually being developed, Windows Defender relies on up-to-date definitions to determine if software that is trying to install, run, or change settings on your computer is spyware or potentially unwanted software.
Windows Defender works with your Windows Update settings to automatically install the latest definitions. For more information, see Change how Windows installs or notifies you about updates.
Open Windows Defender by clicking the Start button , clicking All Programs, and then clicking Windows Defender.
Click Tools, and then click Options.
Under Automatic scanning, make sure the Automatically scan my computer (recommended) check box is selected.
Select the Check for updated definitions before scanning check box, and then click Save.
If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
If you don't use scheduled scans, or if you don't get updates automatically, you should check for new definitions at least once a week. To help protect your computer, Windows Defender will notify you if your definitions are out of date for more than seven days.