Thank you for your help Lee. The modal form sounds like a good idea -
although it will mean that the user interface is different to the legacy
system we are replacing. However as this difference is in a 'helpful' way it
shouldn't matter too much. I have also just tried 'disabling' the form - and
that seems to work too.

However, I have realised that I did not explain the other part of the
problem in my first message. There is one situation in which we would like to
disable the application completely (in which case either of the two methods
you suggested would work fine), but there is also another situation in which
we would like to ignore all key events apart from Alt-S. This is because
there is a hardware switch which has to be set to allow data to be saved to
the remote system. When the switch is set all input apart from Alt-S (the
shortcut to save) must be ignored.

Do you know of any way to do this? I was hoping that if I found a solution
to the first problem using some kind of message filter, it would be
relatively easy to modify to ignore everything except Alt-S. Is this possible
do you think?

Sorry to not explain the other half of the problem (or the other problem -
as in many ways it is a slightly different problem) the first time.

Cheers,

Robin