Spent some time helping out NASA today with a USB smooth stepper/mach 3 problem today.

Background.
New computer to replace old computer.  Both new and old computer were windows 7 enterprise 64 bit.
after getting mach3 downloaded and installed
Latest USB smooth stepper driver and plug in downloaded and installed  (Driver version SmoothStepperUSBDriver2.08.24)

If you launched mach3 and chose printer port as the device mach would effectivly run so you could click on stuff, if you chose the smooth stepper plug in and clicked ok mach would FREEZE right there.  Only way to do anything is to reboot.
I tried a different version of the plug in.... same.

Got the old mach 3 install off of the old computer, renamed and replaced the mach 3 folders  same situation, mach would freeze up.

uninstalled the device driver and installed an older one and it all worked

This is more of an FYI then anything else.

This was reported on the Warp9 forum as well. It seems that the newest drier has issues with some PCs. As I recall Greg had honed in on the problem, something to do with FTDI depreciating a driver function and has been rewriting the plug in to not use that function.