first - you can reset the router back to factory default settings with the reset button (provided it has one, MOST do). You will have to fully re-configure the router after this happens to get it working again.
second - you don't need the WEP key to login to the router. use a wired host to login to the router, and set a new WEP key, and change it on the other computers. If you lost the PASSWORD to access the router, I would suggest a factory reset, and re-configure it, and make sure you keep that info somewhere...
third - you do not need software to connect to the router. The ONLY thing that both wired & wireless hosts will need is the correct drivers for the NIC in that host. Other than that, you won't need software to connect to the router. Some wireless NIC will come with it's own wireless manager to use in place of the Windows manager but it isn't REQUIRED
fourth - for anything software you lost, drivers for the NIC maybe, you can visit the homepage for that device, and find them for free online.