You're probably better off doing the following:
1) In the creationComplete event for the <mx:Application> tag,
a. call a function login().
2) In the login function, popup a TitleWindow with your login fields, and make it modal.
3) When user successfully logs in, close the popup.
Thank you for your reply. With this reply i am not saying that both your solutions are incorrect, rather appreciate the feedback and i may have to use one of the method suggested. However, I am trying to use a stage in the flex builder 3, designer to build the login screen as well as the other screens that will be used. I am sure there is a different way since i have since something similar in the training videos where you defined a whole heirarchy of components and then just name the tag for the new screen. How do i do that and how do i use the flex gui to build multiple screen on the same stage. Or this can't be done the way i think?
you can use view states in your application. To make things more clear and categorised .WYSIWYG manner. States separate content of your application similar to concept of forms and screen but View States in flex are quite more appealing.You should divide your content across multiple View States if it suits design and flow of your application. You should be probbaly making a view state with name 'Authentication' or something . then you can easily move away from this state programatically using the currentState propery of your application and you can also add transitions while moving away from currentSate to another one like wiping conent of the current State in animated fashion. you can get more information about view states at following link
Yes!!! This is what i was looking for! JAZAK.