You can remove Windows Defender by going to Start > Control Panel > Add or Remove programs find "Windows Defender and click on it and click "Remove".
You can also remove it by command from Run.
Click Start > Run copy and paste following command and hit Enter.
msiexec /uninstall windowsdefender.msi /quiet /log uninstall.log
If you still failed to remove it you can download Cleanup Utility from below link.
http://support.microsoft.com/kb/290301/