That will disable desktop composition even if your computer supports it. If you want to achieve this through the command line or through a command script, the appropriate command is REG ADD HKCU\Software\Microsoft\Windows\DWM /v Composition /d 0 /f.
In all seriousness, you can change a registry key to control composition instead of disabling the Desktop Window Manager Session Manager service. :p I suppose they could have called it 'The thinger-majigger that turns DWM on or off' :p It's really appropriately named - it's the Session Manager for the Desktop Window Manager which enables desktop composition. Go start -> run -> services.msc and stop and/or disable the 'Desktop Window Manager Session Manager' service (no thats not a typo, they have manager twice in the name).
