Most of us strive to remain compatible with XP, a few insist even on Win2000. And you can need a speed boost. Check the Lab for fast algos.
All the software I sell is compatible with Windows 2000. There are some places still using it.
It used to be with NT as well, but nobody is using NT these days I suspect.
We have a number of boxes for testing and what is left runs from virtual machines. I have also DOS 6.0, Windows 3.1, 95, 98, Me in virtual machines. If someone wants to test something we can.