Cannot start-up as Administrator

I need to log on as Administrator to be able to insert the WAP key for my wireless connection. Unfortunately, some time ago, to speed-up the operation
I did something very smart (!) to avoid to see the logon window where one
usually reads "user" and "password ". Anybody knows how to go back to the
"normal" sequence of events at startup? Thanks a lot, Paolo from Rome (near
Vatican City).