Whilst I don't use Windows for controlling my cnc equipment I'm a big fan of keeping them off the network and physically securing the machine (which should be the first rule in any security plan).

For what you would pay for Mach4 License ($200 USD) you could get your hands on a Mesa Ethernet FPGA card and run Linuxcnc, a 7i92 Ethernet card (about $90 USD + shipping )will work directly with an existing Parallel Port setup. A suitable machine (if an upgrade is needed) as a start, HP 8000 SFF.

Just a thought and I know many shy away from Linux.

Forum member CliveS is a very experienced Linuxcnc user in the UK. And a decent gent to boot.