Without taking into consideration of security reasons for doing this... This is the most basic way to do this.
I presume the server has a static IP? Add another IP on another subnet that the router does not manage.
On the Windows PC, set the IP to a static on the same subnet as the server's second IP.
The Windows PC "wont" be able to talk to other devices on the same physical network other than the server.
EG, your main network is on 192.168.1.1-254
Set the secondary IP to 10.0.0.1-254
Server has IPs
192.168.1.200
10.0.0.200
Windows PC IP
10.0.0.100
Your router won't route on the 10. network so your Windows PC and Server will only talk to each other and any other device that uses the same IP range.
I do this for CCTV cameras on a home network. It is not foolproof or hackerproof but it works fine if you are not concerned about the security aspect of doing this.
After thinking about it for a while I'm leaning toward just manually switching cables around since it won't have to be done very often. This is only to install and play games on Windows 10.
If this is to play totally legit and defo not cracked games,,,,,,,,,, don't even plug that PC into the network while Windows is running.