You can move all your axis to an extreme and press the Ref all button, make sure you dont have any of the limits enabled in ports and pins though as your axis will start moving trying to look for them. When you have done that go to Homing and Limits and set the X and Y min as Zero and the Z max as zero. Now if you know the travel of your axis set the dimensions in the X and Y max and Z min. If not jog each axis to the other extreme and make sure you are viewing the Machine Coordinates (press the Machine Coords button so RED LED is showing) Take a note of these axis readings and enter them as your X and Y max and Z min.
Every time you start Mach you will need to move to your axis extreme and press the Ref All button so that Mach knows where the table is. You can enable your softlimits (button on main page below axis DROs) you should now be able to jog towards an axis extreme and Mach should stop before it gets there. You can set up a sow zone so that Mach has time to decelerate before it gets there, my machine has a 5mm (0.2" approx)slow zone but it will depend on your machine how small or large you make this .
Yes Mach is for either Windows 2000 or XP, you can also use Vista but Vista is ............ in my opinion
Hood