I enabled it with the Control PanelUsers applet, but now I want to go back to how it was before, where it assumed one user for the machine and didn't force the login dialog.

I tried deleting the user I had set up, but I still get the dialog box, with an empty list of users. I can cancel out of it and get onto the machine, but the box still pops up.

Tried TweakUI's "auto-login" - didn't have any effect.

There's got to be a Registry key to fudge, or some other procedure, to get it back to "assume one user for the machine" mode.