You'll need to set up a different static IP address for each new computer that you want to port forward.
PortForward.com - Free Help Setting up Your Router or Firewall

So go ahead & do that for the new laptop, then set up your port fowarding & use a different port to the one the original computer's using (port 64049 is a good one to try, or something in the range of 49152 - 65535).
Port Forwarding for the Linksys WRT54G

Make sure that LW's listening port, port in the 'manual port forward' box & port forwarded within the router are all the same number

Even though you're seeing the firewall icon, is LW fully connected (no red connection bars) on the new computer?