Is this the Regedit you used ?
Click on the Start menu and select Run (Or press Windows key + R). You will get the Run dialog box. Type Regedit and hit Enter. The Windows Registry Editor will open. Navigate to the key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Microsoft Security Essentials
and delete it. After deleting the key, reinstall Microsoft Security Essentials and try to remove it using Add/Remove Programs again. If it is a success, you are good to go.
IMPORTANT: The above involves making changes to the Windows Registry. Make sure you create a system restore point before working with the registry. You can also backup the entire registry using ERUNT
fed up with Windows antics...??
LINUX IS THE ANSWER
I USE LINUX MINT EXCLUSIVELY... NO DUAL BOOT, NO VIRTUAL MACHINE
Failure is not an option. It comes bundled with your Microsoft product.