SDI interface and modal forms

Imagine, if you will, this scenario:
 - An application has three forms, Form1, Form2 and Form3
 - A startup, Form1 opens in a maximized windowstate.
 - From a button on Form1, you can open Form2, which also is in a
maximized state. The user will want to be able to switch between Form1
and Form2 with ALT+TAB. So far, so good.
 - From a button on Form2, the user may start Form3, which is opened as
a modal dialog with ShowDialog().
 - And now, for the tricky part: the user decides that he/she wants to
do something in Form1 before returning to Form3, and presses ALT+TAB.
This brings up Form1, but none of the buttons on it are clickable, since
the application is in a modal state waiting for Form3 to be closed.
This, however is not evident to the user, which cannot be expected to
grasp the concept of modality when Form3 is not visible, and may as such
be classified as "user hostile" behavior.

So, the question is: how can you prevent forms from being accessible via
ALT+TAB or the Windows task bar when a modal dialog is open?

Was that at all understandable?

Thanks,

Eyvind.