The problem is that when you press enter in the password box, you aren't pressing the button. Presumably you are performing the login action when you click the button.
You might try having the login process happen when the password textbox "OnChange" event fires. Seeing as this is a password box (and always starts out blank), the OnChange event should always fire (I hope).
Otherwise you could trap the enter key on the password box. There is a recent thread