The fact of the matter is that it's just not possible to do what Mach3 does on a 64 bit OS.
WHAT?
That doesn't make a bit of sense. How on earth would a 64bit OS not be able to handle what a 32 bit OS can do?
Mach3's driver runs at a low level in the OS, very similar to a virus, and actually has some control over Windows. Microsoft has made changes to the OS that make this no longer possible.
It's not just some standard Windows program we're talking about. Mach3 needs to send up to 75,000 pulses, every second, to each of up to 7 axis, without Windows getting in the way.