I get a specific message from Windows XP pro when I try to enter standby to the effect that "The driver from MACH3 is preventing standby" etc and saying the MACH 3 should be closed and try again. So my assumption is that it's the same situation with your full shutdown/hang -- which I also experience. I am fortunate in that my machine was rebuilt from scratch before I loaded MACH3 a few weeks ago and I have few other programs on the machine -- but the problem existed when the machine was almost totally free of any other software. Otherwise it could become a fingerpointing exercise on a fully loaded machine.
I suspect that this is easy to fix, but that the MACH3 team is doing much more productive work making their outstanding product even better. I really can't complain, but maybe it will get fixed somewhere down the road. As Hood said, ' just close MACH3 (like we should) before shutting down'.