I ran that exact setup, never ran into that issue.. There don't seem to be any options in the either BlackBerry device driver refering to interfering with any power state.
Would have any objection to un-installing 4.3 and moving to 4.5?
Or if you want to try this:
Right click "My Computer" hit "Manage" expand "Event Viewer" and go into the the "System" log in there you find an "Error" refering this issue where the computer can not go into Stand By, if you find it copy and paste it here, there might be something in there that can point us to the cause.
Of course those steps may be more on the advanced side you want to try moving up to 4.5 first that might be easier for you.
8830 -> 8330 -> 9550 -> 9650
Just think about how far BlackBerries have come from then till now... And what else is coming.
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