I wouldn't call them business-critical computers, but I do have about 3-4 computers under one of my workbenches dedicated to certain work I did for a client. One of them has an EPROM programmer set-up, which includes a custom ISA-card, and a ribbon cable to an external box with a ZIF-socket. The software for that requires MS-DOS. To network that box, which is a 486SX, I run WFWG 3.11. Since these never go on the enternet or anything outside 'the lab', I don't worry about the old MS networking.

A second computer has an MS-DOS program that captures data input on a pair of RS-422 ports (two more special ISA cards) That let me look at packet data at a low level and watch which serial port received what data, and track character-level handshaking of a proprietary protocol.

A third computer under Windows 2000 ran a 68HC11 cross-compiler that didn't require a dongle. I also used this to run a remote debugger that allowed me to single-step through the 68HC11 code running on the target but moved to the target's RAM. To do this 'properly' I was constantly generating hard-copies from the compiler which would show me the C-code, and the resulting assembler code that I'd track through the debugger. I used a lot of paper on a project sometimes.

A fourth computer with MS-DOS, with WFWG3.11 also installed on it, had all my old CAD programs on it, from before they required dongles, so that I could pull-up drawings of some of the hardware I designed. Some of those were finicky about which mouse I use. I still have an old Logitech mouse C7 that two of my CAD programs required.

I was working on a replacement platform that put this into an Atmel-based controller, shipped a couple of samples, then went blind in one eye, while my client had a stroke and was out of action for a year. Between both of our health problems, that project kind of died.