I have had a few production machines that were controlled by a computer running Windows NT 4.0, and the machine (which costs over $120,000) could not be controlled by newer software because... well... the company that made the SOFTWARE part for it, no longer exists. The hardware is still maintained, but the machine is obsolete (20 years old), but would cost almost $200k to replace in today's dollars.
Since the maintained production machine (cutting metal plates into pre-designed pieces) will be just fine for another 20 years (it just needs grease to operate, and a computer with a COM port and Windows NT 4.0), we have an old machine on both machines (we had 2 of them) running Windows NT 4.0. When I left the company, they were Pentium III computers with 512 MB of RAM.
The LAN ports those computers are connected to, where configured to be in their own VLAN, with a firewall allowing them to have access to ONE folder on a network server only, no internet access at all. That one folder was needed so the engineers could place the designs. (the file structure for the designs is still used to this day, so the new software used by engineers designing the stuff on their big fancy 30" screen flat panel quad core beasts, could still be used by the machine running the Windows NT 4.0 software.)
This all said.... if you do have software that can't be upgraded, there are ways around it, as long as you take the proper security precautions.
I would, however, NOT recommend this procedure for regular, internet accessible machines used for every day computing.
