myrea,

I am assuming you want to let other users use the computer while you're away just not your app.

You could hide the main form (form1.Hide) then show it again after sucessful login (form1.Show).

Instead of putting up the login screen you could use a task tray icon. When the user clicks on the icon he gets the login window.

This way you won't have to worry about what happens if he closes the login window because you can bring it up again from the task tray icon.