Ryan,

First I would suggest that you make sure that you have set the Tab Order for
the controls on your form.  This will insure that your users are navigating
throught the options on your form in the way that you want them to.  Doing
this may solve your issue altogether because when the user enters their User
Name and presses the enter key they will be moved to the Password field.  
Like wise when they have entered their password and pressed the enter key,
the focus would be passed to the Login button and user could just press the
Enter key again and the code assigned to the OnClick event of the Login
button would be run.

Now, to answer your question;  I assume that want you are wanting to do is
to fix your form so that the user does not have to click the Login button
with the mouse because pressing the enter key is actully an indication to any
control that you have finished your entry into that control.  You can set the
Login button to be the default button by setting the Default property of the
Login button to Yes.  However, be aware that this is really designed to be
used when you have multiple buttons on one form.