This may sound like a silly question, but have you verified that the destination computer's IP address is actually 192.168.1.101 (Linksys DHCP starts with .100 by default) and that the port is open on the Windows (or other software) firewall on the computer? It's entirely possible that the router is forwarding the port exactly as you instructed it to, but it's not going to the right computer on your network or it's being blocked by a software firewall.
Second, is your ISP's "modem" a simple pass-through device or does it have its own internal NAT firewall/router? If the ISP device is a router, you'll need to forward the port from the modem to your Linksys router's WAN IP address, and then from the Linksys router to your computer's IP address.
Also, as drebo mentioned, your ISP could be blocking the traffic, and not just that port but P2P traffic in general. If this is the case, there's really not much you can do about it except try another port.