adding custom events to word 2007 built-in buttons

[aanchal]

Hello,
I am making a plugin for word 2007. I want to attach an event to the
"PageSetUp" Gallery so that a custom action is performed before the default
action.
The position of the gallery is "PageLayout->Page Setup->Orientation".
I want this custom action to be called whenever the orientation is changed
from Portrait to landscape or vice versa.

Thanks,
aanchal