I'm not sure why you think the current machine can not be controlled by a newer PC. If you are concerned the new PC won't have a serial port, just purchase one:
http://www.amazon.com/StarTech-com-R...erial+pci+card
I think the primary problem here is that you have not clearly explained your problem. There are so many work arounds to the issue you are describing, but you keep shooting them down. Perhaps you should tell us more, so we can assist with something that will solve your problem.