It is the operating system being 32 or 64bit that is the issue. You could install 32bit XP on a new computer, assuming you can get drivers from the computer vendor for XP.
If using a 64bit operating system to actually control a machine then you must use an external controller to control the machine with Mach. That can be Smoothstepper (USB or Ethernet), CSMIO/IP, Galil, DSPMC, Kflop etc etc
If just wanting to use Mach in simulation mode, ie no machine being controlled, then you can run Mach on a 64bit operating system. As Terry mentioned some things will not work correctly in simulation, these things are the functions that use the driver or plugin such as probing. Otherwise it will work fine for testing and proving code.
Hope this helps clear things up a bit.
Hood