You dont need the software for the router. IMO it can cause more problems then its worth. You can get into the router by its IP. If you check your LAN connection it should be the default IP (been awhile its either that or one of them).
Is the place you can download the software. Can also get there by typing in linksys and going to the companies website (cisco).
Then click on customer support link under Need help.
Near the upper right corner there is a search bar. Type in wrt54g and select your router. Will have a tab to download the correct software.