Open the control panel. Open "user Accounts". Click on change the way users log on and off. Uncheck use the welcome screen. Now logoff. Instead of seeing a list of users to login as, you will get a Username/Password box. Use administrator as the username and your admin password. You can always turn the welcom screen back on later.
Edit:
I may have misunderstood your question. Anyhow, when on the screen with use the welcome screen you will see use fast user switching. Check this off and you should be able to switch user.