Ouch!!! Your Linksys WRT54G is a known "lemon" that Linksys released. It has firmware problems that Linksys never bothered to fix. Thus UPnP is highly unreliable when using p2p or other heavy bandwidth programs.

To improve reliability the router should be port forwarded.

Steps must be in this order & is vital:

1. Set up a Static ip: Static ip for Windows XP instructions

2. Port forward Router: PortForward.com - Free Help Setting up Your Router or Firewall (find your model in column 2 near bottom of page)

3. Settings within LW will be described within step 2 above. If not see Manual port forward instructions & sample image


(BTW there is a 3rd party fix to the Linksys WRT54G firmware but I do not recommend it unless you are sure what you are doing, else you can render the router workless.)