A USB-to-parallel adaptor is completely unusable for CNC. It is impossible to generate the precise signal timing required. You must have a real parallel port, A PCMCIA parallel port should work OK. The next problem is many laptops share the main memory with the graphics controller, which can also cause problems that lead to unreliable operation. Power management also causes problems, though this can *usually* be disabled in software.
If you run Mach3, you can get a SmoothStepper, which is a hardware motion generator that connects to the PC via USB, and works very well. But, sounds like you machine uses dedicated software which would not know how to communicate with a SmoothStepper.
Your best bet is to listen to the machine manufacturers recommendation. To do otherwise is asking for trouble.
Regards,
Ray L.