Maybe I missed some instructions somewhere, but my machine has been acting erratic and generally not working. A big clue was my PWM frequency not being correct. Turns out there's a setting in Vista under control panel, power management, advanced power settings. Expand the tree and find "Minimum processor state". Make sure this is set to 100%. Also do the same for maximum.

Once I did this, my PWM frequency is correct and things work better than ever!

Mach should probably do some kind of loopback test or check for these settings in the registry. This would have saved me weeks of pain. Hopefully this tip can help someone else.

Were you then able to use a USB - parallel adapter?

Anyone know if Mach 4 will work with USB? Or we stuck with a parallel port?

You can't use a USB to parallel adapter, it will not work (they are only for printers.) There are a few different choices if you want to go USB. My favorite is the SmoothStepper: http://www.soigeneris.com/SmoothStepper-details.aspx (my website), there is also the DSPMC, and KFlop boards.