If it is a PPt controller, one solution could be to use a UC100 motion controller. This plugs in to the standard BoB parallel port input and interfaces to the PC via USB. I have two, on lathe and mill, and they work very well.

I think you are saying that your PC doesn't have a parallel port, but if you are short of USB ports how about adding a cheap hub on one of the ones you have to get 3 or 4 extra ones so you can liberate one of the ones you have for the machine connection?