First time here. I am building a 17X20 overhead leadscrew CNC machine following the plans from Solsyla. I will be using the 3-Axis ProboStep Stepper Motor Driver Kit and a trim router. I found out that new computers usually don't have parallel ports that are needed by Mach 3 Mill software. Which operating system is best for Mach 3 Mill software? Any tips on using Mach 3 Mill software with the 3-Axis ProboStep Stepper Motor Driver Kit?

The smooth stepper should be out in the next few weeks, this will do away with the need for a computer with a parallel port. Your other option would be to use a PCI Parallel port.
 Win 2K, XP or Vista can be used, personally I wouldnt touch Vista with a 10ft barge pole but thats maybe just me Once servicce pack 2 comes out for Vista I may try it but from what I have seen of it so far on friends computerss I have helped set up its a POS.
 No experience whatsoever with your chosen drives so cant comment.

Hood

Where do I get info on Smooth Stepper?

Where do I get info on Smooth Stepper?

http://www.warp9td.com/index.php is the website, there is also some info further down in this forum.
Hood

I'm not sure about USB because a lot of devices can be connected to USB ports. I think that if you have too many devices plugged into USB ports, there may be problems with the controller for the CNC machine.

I have been Beta testing the smooth stepper for 2 or 3 months, have a keyboard, touch pad, touch screen, wireless network adapter and the SmoothStepper all in USB and have not had a problem. I think there are over 20 Beta testers and as far as I know no one has had such a problem.
 If the Smooth Stepper had a communication problem then it would fault mach with an error so dont think there will be issues.
 To be honest I was a bit sceptical like you are, because I have had printers, scanners etc connect and disconnect when on USB but there doesnt seem to be that problem with the SmoothStepper, maybe Greg or some of the more technically minded can give you more info or assurances.

Hood

What's the system requirements for the Mach 3 Mill software?

Just a quick note about USB:

USB is a very capable standard (127 devices per host controller), so there's no device issues related to multiple devices on one host controller, but still it would be best to not attach devices with high bandwidth needs (ie- USB external hard drive, USB audio or video device, etc.) while machining even though USB bandwidth is very good in USB 2.0. Other than that cable runs are 6' though with an active line amplifier/booster 20' is possible.

Featurewise, a downside compared to LAN is no wireless. Another, is of course, the short cable runs. LAN is better and the motion-control industry has choosen it over RS-232/422 which is quickly becoming legacy.

http://en.wikipedia.org/wiki/Universal_Serial_Bus

USB will go wireless soon though, it'll be called USB-IF and you can read more about it here: http://www.usb.org/developers/wusb

-Haik