When ender` originally reported the problem, I tried configuring APM (Advanced Power Management) for an NT4 guest to see if that was a possible workaround, but I couldn't get that to work. APM itself was functioning, because my test programs could happily do an APM power-off of the VM, but NT4 was not happy with it for some reason. I didn't think to try an older hardware version, though... Interesting...
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Darius