Phil - I run Mach 3 through a lap-top - Toshiba, about 5 years old with USB 1. I use the standard LPT1 port to run the lathe/miller. I have never had any problems, it runs the system superbly, and being a laptop, sits on a shelf out of the way.
I got a web camera for Christmas - and plugged it in. I got a picture through windows, I couldn't get a picture through the video window on Mach 3, the system started running slowly, it told me I had a fast (series 2) USB device in a slow USB slot, and finally the whole lap-top crashed, I lost the screen picture etc etc. The memory seemed to get over written.
I gave up with the Web Cam and all is back to normal.