I think you're out of luck. Hindsight is 20/20, but shouldn't their hardware and software requirements have come up long before you started development? How did you get to the point at which you were actively developing without knowing your target environment, especially if you're on premise?
At first I wrote a response suggesting a few ways you might be able to get it working on older versions of Windows, but they're just not feasible for anything prior to XP. Possible? Probably, but more work than rewriting everything, most likely.
You might be able to convince them to upgrade and virtualize their Windows 2000 programs--heck, you might even be able to get away with using DOSBox. That seems a lot more realistic than virtualizing Linux or a newer version of Windows on Windows 2000. But I wouldn't get your hopes up--anyone still on Windows 2000 is going to object heavily to upgrading; that's why they're still on Windows 2000. You can eliminate all the barriers for them and they still won't want to do it.
Edit: This might sound crazy, but have you looked into whether it's possible to make some changes to Electron to get it to run on Windows 2000? It's designed to be portable, so, in theory, you should be able to get it to run on a potato--assuming said potato has enough RAM.