My wife suffers from MS and is bedridden but still retains some movement in
her hands. I have just built a system to give access to the internet,
telephone system and DTV broadcasts via a modified mouse and large,
flat-panel screen.
In the second phase we want to install an alarm function with a variety of
triggers. One of these triggers will execute the alarm function when any
mouse button is held down for four seconds (the alarm function includes the
appropriate cancellation processing). This trigger needs to be available at
all times.
Is it possible to modify the mouse driver or a background process to do
something like this?

If MouseButtonDownEvent then
Set Alarm to True
Set SoundAlarm to (Now + 4 seconds)
Loop until SoundAlarm = Now
HandleEvents
(end of loop)
If Alarm is True then
Execute Alarm Process.
If MouseButtonUpEvent then
Set Alarm to False.

Please excuse the presentation but I think it's clearer this way.