Picking up a 404 error in a web browser control

I wonder if anyone can advise me of the best way to pick up a 404 error in a
web browser control on a windows form.

By way of some background the control is being used to display nethelp for
the application. In those instances when the server is down (and therefore a
404 page not found is displyed) I'd like to pick up the error prior to the
standard 404 page being displayed and redirect the browser control to a local
source of the help files.

I'm using vb.net in visual studio 2005.

Many thanks for your thoughts.
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Dom