You're right... the P port is pretty much gone in most newer computers. Still, even if it involves buying a PCI card to have one, the parallel interface represents the cheapest and simplest solution for a software based CNC controller. And isn't cost the main reason for which MACH exists in the first place? Small machines are getting more and more common and affordable for even the average hobbyist and MACH should be ready for this - an OS agnostic, no fuss install, without the "making it actually work" stage. A user interface overhaul should also be a priority, but that's beyond the scope of this thread.
I don't want to question ArtSoft's programming skills by any means, however if 64 bit systems and their specific quirks are not an area of expertise, then I believe a little outsourcing would be in order. And if that's the only reason for which I'm in the situation described in the OP, I'd be happy to provide some assistance.