Microsoft Security Essentials Removal Tool uninstalls Microsoft Security Essentials if you cannot uninstall it in Control Panel by using the Add or Remove Programs item or Programs and Features.

Windows XP
Click Start, click Run, type appwiz.cpl in the Run text box, and then click OK.
Select Microsoft Security Essentials, and then click Uninstall.
Restart the computer, and then go to the "Step 3: Reinstall Microsoft Security Essentials" section.

Windows Vista or Windows 7
Click The Start button and in the Search programs and files text box, type Appwiz.cpl, and then press ENTER.
Right-click Microsoft Security Essentials, and then click Uninstall.
Restart the computer, and the go to the "Step 3: Reinstall Microsoft Security Essentials" section.