Or you can try disabling User Account Control, since that seems to be the foundation for Defender. However, doing so may weaken the security of your system. Instructions at
http://www.petri.co.il/disable_uac_in_windows_vista.htm.
You can also disable Window Defender's Real-Time Protection by:
* Open Windows Defender
* Click Tools
* Click General Settings
* Scroll down to Real Time Protection Options
* Uncheck Turn on Real Time Protection (recommended)
* After you uncheck this, click on the Save button
* Close Windows Defender