Can you give some idea as to when this started? I.E. immediately after a windows update, Rhino update or madcam update?
I just recently upgraded my computer to a new I7 quad with 64 bit Win 7 and had a problem on the initial install, but not since.
Have you tried a complete uninstall of Rhino and Madcam then reinstalling them back at the same time?
If this doesn't work, please send Joakim an email with your ph number and a time to call along with a descricption of the problem. If you can do a screen capture that might help as well.