Keep Windows Defender definitions up to date

Applies to all editions of Windows Vista

This is from Microsoft. Pasted here to avoid broken links.

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 unwanted or malicious 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.

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

1. Click Start
2. Click All Programs
3. Click Windows Defender
4. Click Tools
5. Click Options
6. Under Automatic scanning, make sure the Automatically scan my computer (recommended) check box is selected
7. Select the Check for updated definitions before scanning check box
8. Click Save
9. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.

To check for new definitions manually

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 To help protect your computer, Windows Defender will notify you if your definitions are out of date for more than seven days.

1. Click Start
2. Click All Programs
3. Click Windows Defender
4. Click the arrow next to the Help button
5. Click Check for updates
6. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.