Dude... for real? You can setup your modem to do all the user/pass stuff for your ADSL setup then set it to hand out IP Addresses via DHCP. If this isn't possible, buy a simple router and set it up to use your username/password for the DSL setup. There is no reason you should *HAVE* to use the computer to authenticate for your ADSL setup. I never do that, its a pain and a waste. I have 4 machines in my house and I sell them and build new ones all the time.
I'd hate to have to re-setup my ADSL connection over and over. Pending on what type of modem your have common usernames and passwords could be
just go to whatismyip.com then take your IP address and insert it into your browser address bar and your should be at the login page for your modem.
If you already use a router login to your router using your gateway ip address your machine has. you can find your gateway using the command "netstat -nr" in a terminal. They are usually
When you get there complete the WAN setup. on My Belkin N+ its under "Internet WAN" and the setup option your looking for is "Connection type" and your are possibly using PPPoE .
Just go from there dude. good luck