admin user name not recognized

When I try to install an app, restart, or other things that require an admin, I put in my user name and password and it says "You must enter a valid user name." I tried changing my user name, and it still doesn't accept it when I type it into the dialog box. I know it's the correct name and password - the name is displayed right there on the screen, and the password works fine for logging in. And my account does have admin privileges. Just somehow it refuses to recognize the user name when it requests it.