Putting in a router is kinda overkill. You can restrict access very easily in the windows firewall on the machine and set it to not allow and communication with other devices in your network other than the main router.
You could I guess buy a hardware firewall and do the same thing if you really wanted to.
A simple router only to a point will block the access because of the nat. You would still have to put in firewall type rules to really block all the traffic.
Because it makes things so complex to put in another router I would try the simple solution of firewall rules on your client machine and see if it will meet your needs.