http://answers.microsoft.com/en-us/protect/forum/protect_start/uninstalling-mse/a63b8c4b-58ed-437e-8086-fa08d80725a4


There are two manual removal methods that you can try:

 

Alternate solution #1

 

The following information was provided by forum user, Aiscer, to assist in removing MSE if using Control Panel/Programs does not work properly:


1. Download the MSE installer for your Operating System from the Home Page:

 


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Note: Editing the registry incorrectly can cause your system to become unusable. Proceed with caution.

Use the Registry Editor to delete the following key from the Registry.

On XP, go to Start, then Run, and type:

    Regedit.exe

Press Enter

On Vista/Windows 7, in the Search box above the Start "orb" type:

    Regedit -don't press enter.

Right click regedit.exe in the results list above and select, "Run as Administrator"

 

Navigate to:

     HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Microsoft Security Essentials

by expanding the branches in the left pane until the branch forMicrosoft Security Essentials is displayed.

Right click "Microsoft Security Essentials" in the left hand pane and select Delete.

Close the registry editor.

 

Then:

 

XP:

Click on the Start Menu, go to All Programs, Accessories, Command Prompt

A “DOS” window opens

Assuming that your system drive is C: -

At the prompt, type:

     CD “C:\Program Files\Microsoft Security Essentials\Backup”

Press Enter

Type:

    setup.exe /u

Press Enter

Setup will launch and will uninstall MSE

 

Vista and Windows 7:

Type command in the search box (above the Start “orb”), do not press enter

In the results list above, under Programs, right click Command.exe and select "Run as Administrator"

Approve the action via the UAC prompt, if it appears.

A “DOS” window opens.

Assuming that your system drive is C: -

At the prompt, type:

     CD C:\Program Files\Microsoft Security Essentials\Backup

Press Enter

At the prompt (c:\Program Files\Microsoft Security Essentials\Backup>), type:

     Setup.exe /u

Press Enter

Setup will launch and will uninstall MSE