This happens to me alot when my Dell laptop w/ Vista is restarted. It seems to be the bluetooth-- Vista says the bluetooth sent an error and the driver was not installed. Usually a reboot fixes it. I suggest once it is working don't turn your PC off!
Try new batteries. If this does not work then try reinstalling the blue tooth drivers. If this does not work then replace the mouse. if you purchased it from Dell, it might be still under warranty.
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