want to change back to single user login

I used to have just one login popup at login time, and all I had to do was enter the password. This was for a user account with administrator privileges. I somehow (forgot how) managed to add Administrator account to the login screen, so now I have to click on the user account I want before typing in the password. Now I want to change this back to the single-user login, because I don't use the administrator account often enough to be bothered with it; however, I still want to be able to use it if necessary. Is there some simple way to switch back and forth between these two modes? I have gone over the control panel options with a fine-toothed comb, but can't find anything, and I really don't want to get into regedit if I can avoid it.

Thanks for the very useful advice and links. That was a piece of cake, and I will make a sticky so I'll know what to do next time.