What router is it? Usually in the manual there is a default username and password. To access the router you must have it's ip address. To do this goto start>run type "cmd" (without quotes) then type "ipconfig" again without quotes. Look for default gateway, it could be 10.1.10.1 or 192.168.1.1 or something like that depending on your ISP.
Now, go into internet explorer and type that number in, you should be greeted with a login screen, type the default username and password in the login boxes the key should be somewhere in there.
They WEP Key should be 8 for 26 characters if their is an 8 AND a 26 then choose the 26 and type that into the nokia. Good luck,