Virus and spyware definitions are files that act like an encyclopedia of known malicious software, including viruses, spyware, and other potentially unwanted software. Because malicious software is continually being developed, Microsoft Security Essentials relies on updated definitions to determine if software that is trying to install, run, or change settings on your computer is a virus, spyware, or other potentially unwanted software.

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

  1. Click Settings, and then click Scheduled scan.

  2. Make sure the Check for the latest virus and spyware definitions before running a scheduled scan check box is selected, and then click Save changes. If you're prompted for an administrator password or confirmation, type the password or confirm the action.

To check for new definitions manually

  1. 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example, if you didn’t turn on your computer for a week), Security Essentials will notify you that the definitions are out of date.

  2. To check for new definitions manually, click the Update tab and then click Update.

While updating definitions, Security Essentials displays an "updating" icon in the notification area.