Wendell is likely correct. First, the router's firewall is designed to keep outside traffic from accessing your network. You shouldn't need to do any port forwarding.
The router is likely also your DHCP server and is assigning IP addresses to all of your computers. This is probably a different subnet than before. If your network were set up correctly before, you would have had a static IP on "main computer" with the others receiving their IP configuration from the router.
I would go to any computer and bring up a command prompt: Start -> Run -> type "cmd" without the quotes. Then at the command prompt type: ipconfig /all
This will show you the network configuration for the client as well as for the server. If they are on different subnets and/or the DNS are not pointing to the server IP, you will likely have issues.
If you need to do anything with firewalls, it would be with the server and the clients. The best way to do this is not to add things such as computers and ports but to just turn them off and see if you can connect. If you can, then you KNOW it is the firewall, and you can make the changes.
If you do not wish to do it this way, you can turn DHCP off on the router, and set static IPs on the clients, but DHCP is always better.
