ok well did you install any software yesterday? if I were you I would check and see what did it and make sure its not a virus or something.
other then that to totally remove the welcome screen you need to do this:
1) Click on Run
2) Type in the following command and click OK
control userpasswords2
3) Highlight the user you want to log into automatically, then uncheck the box for
"Users must enter a username and password to use this computer"
4) Click on Apply and you'll be asked to verify the username and password to log in automatically
5) Click OK and the next time you restart your computer, you'll automatically be logged in without having the classic prompt or Welcome Screen.
good luck