This is a Catch 22!!
Since the Control Panel route does not work, and neither does the Command issued, please try the following:
Looks as if you downloaded the MSE installer, and saved it to the Desktop.
Open the Command Prompt by right-clicking and selecting: Run as Administrator
At the Command Prompt, type (or copy/paste):
Press: Enter
Type (or copy/paste):
[Note!! A space between .exe and /U]
Press: Enter
Any luck uninstalling Microsoft Security Essentials?
If yes, restart the computer, and try to install MSE again.
If still no-go, can you open MSE, go to the down-arrow next to
Help (Upper right) and select:
About
What is the AntiMalware Client Version?