Turn Windows Defender real-time protection on or off

Applies to all editions of Windows Vista

This page is from Microsoft. I pasted it here to avoid broken links.

To help prevent spyware and other unwanted software from infecting your computer, turn on Windows Defender real-time protection and select all real-time protection options. Real-time protection alerts you when spyware and other potentially unwanted software attempts to install itself or run on your computer. You are also alerted if programs attempt to change important Windows settings.

1. Click Start
2. Click All Programs
3. Click Windows Defender
4. Click Tools
5. Click Options
6. Under Real-time protection options, select the Use real-time protection (recommended) check box.
7. Select the options you want. To help protect your privacy and your computer, we recommend that you select all real-time protection options.
8. Under Choose if Windows Defender should notify you about, select the options you want.
9. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.