Hi I have a Linksys WAG54G also.
Try running a command prompt, under programs\accessories. Then enter IPCONFIG
This should show you some information regarding your IP address and your default gateway. The default Gateway IP address will be the IP address of your router. Type that address into IE, and you should be asked for a user and password.
Type the user and password that was originally setup then, you should have access to the routers setup pages.