Make a movie clip for your buttons with your Login button on one frame and your My Filespace button in a second frame. This movie clip must be a symbol so create it using 'new symbol'. Make sure to add a 'stop' action to each frame of the clip (frame, not button).
Now, find your new movie clip in the Library pane. Drag it to the stage and position it where you want it. Select it and give it a name in the instance pane. It must have a name so you can address it using actionscript.
Once you've done that, you can use the 'telltarget' action on the button click. Add the telltarget action, enter the path of your movie clip, and then use a goto action to move the clip from frame 1 to frame 2, and vice versa for your second button. Refer to your 'Help' section for specifics on the telltarget action and specifying the path.
You can use the same movie clip and the same actions from any layer on the stage. If you want to extend the buttons across scenes and loaded movies, just make it a separate movie and load it to its own level.