[This version of Windows Defender is still in development, so some Help information might be inaccurate or missing.]

Keep Windows Defender definitions up to date

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 potentially harmful.

Windows Defender works with Automatic Updates in Windows to automatically install the latest definitions. To make sure that Automatic Updates is on, click Start, click Control Panel, and then click Automatic Updates.

To automatically check for new definitions before scheduled scans (recommended)

To complete these steps, you must be logged on as Administrator or be a member of the Administrators group.

Open Windows Defender. (Click Start, click Programs, and then click Windows Defender.)
Click Tools, and then click General Settings.
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 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 To help protect your computer, Windows Defender will notify you if your definitions are out of date for more than seven days.

To check for new definitions manually

Open Windows Defender. (Click Start, click Programs, and then click Windows Defender.)
Click the Down arrow next to Help, and then click About Windows Defender.
In the Windows Defender dialog box, click Check for Updates.