You can't uninstall Defender, but you can disable it. In the start menu search box, type defender. Click on Windows Defender in the results list.
If you get a prompt that says Windows Defender is turned off, click Close and then head over to this thread:
http://social.microsoft.com/Forums/en-US/onecareupdate/thread/297455ff-0484-4dd6-9627-ca1b0e30b015 and scroll down to the answer.
If Defender opens up, click on Options and scroll all the way down to the administrator options. Uncheck the box to use Defender and then Save. This will turn Defender off.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator