System Error 5 even as Administrator

Question

I'm having some problems** with Microsoft Security Essentials on my Windows 7 machine. I'm therefore following the guide on this page to uninstall it.

I get to the second heading under Step 2 ("Stop and disable the Microsoft Security Essentials service") before running into problems. Specifically, even after opening command prompt as Administrator (and checking that my account has admin privileges with whoami /groups and net localhost Administrators), I can't run the command net stop msmpvsc without getting a System Error 5.

How is it that even as administrator I don't have permission to run this command?

** The problem is that defender is running a regularly scheduled scan every week even when I've asked it not to. These scans get midway through before hanging forever; they never complete and they eat my cpu. I've uninstalled MSE through the native system uninstaller and even though it's disappeared from the installed program list, all its files are still in C:\Program Files\Microsoft Security Client and it still does its thing. I've even tried the brute force method of deleting the Microsoft Security Client folder directly but can't give myself permission to -- maybe this is a related problem?