- https://legacyupdate.net/ - great for Windows 2000 and XP in particular
- https://windowsupdaterestored.com/ - targets more Windows OSes
I have a Pentium 3 system under my desk for running some software that needs access to specific hardware (so I can't just run it in a VM), and Legacy Update was fantastic for getting Windows 2000 working properly.
I've tried ones for 98, 2000 and XP and I've found that most of those unofficial updates go too far - they tend to try to do things like backport DLLs or APIs for better compatibility with newer software, but I've found they make the system less stable and less period correct - which affects software compatibility.
In my case, I'm trying to run a mix of old programming software and old games - both of these had problems on unofficial update packs.
Most network traffic to it is via a samba server on Linux configured to be accessible to SMBv1 still for use as a file dump.
It doesn't really talk out to the internet much - you can't get modern browsers and even if you could it would be too slow to be practical.