Microsoft started bundling Windows Defender with Vista, and continues to do so with Windows 7. It's a real-time antivirus program, running in the background at all times and checking for malware as you download and open files.
A quick note on definitions: Some people will argue that Windows Defender is an anti-spyware program, not an antivirus one. That once meant something, but not anymore. Today, any such program that doesn't look for all known types of malware is useless.
But before you consider removing Windows Defender, make sure you have another real-time antivirus program installed and running. Your options run from full-blown suites loaded with extras to slimmed-down freebies that still get the job done. Once you have one of these installed, Windows Defender is pretty much irrelevant.
Although you can't uninstall Windows Defender from Vista or Windows 7, you can turn it off. To do so, click Start, type defender, and click Windows Defender. Click the Tools icon at the top of the window, then the Options link.
The rest of the instructions vary with your version of Windows:
Vista: Uncheck Automatically scan my computer (recommended), then scroll down and uncheck Use real-time protection. Click the Save button.
Windows 7: Click Automatic scanning in left pane. Uncheck Automatically scan my computer (recommended). Click Real-time protection in the left pane. Uncheck Use real-time protection (recommended). Click the Save button.
Read the original forum discussion.