I've explored trying to run a circa 2008 windows server in something QEMU on a Pi, so I have keep my home Active Directory running, but not have the hungry power requirements of a larger server. After a lot of research I came to the conclusion that Win32 under QEMU on a Pi is just not feasible as even the Pi4 isn't fast enough.

Looks like you can persuade Windows 10 to run on a pi4, even if it’s not supported, so I guess there’s a decent chance even recent versions are runnable too.