This is a simple process really. I'm not familiar with your router/OS but here are some generic instructions:
Setting up bridge mode: Dial into your router (enter the IP in your browser). Your router has an IP address, username and password. Your router is most probably in PPPoE/PPPoA mode. Set that to Bridge mode. Save and exit.
Setting up PPPoE accounts. On XP go to Network Connections, create new connection, connect to the internet, set up my connection manually, connect using a broadband connection that requires a username and password, enter the ISP name (you can call it anything), fill in the username and password and finish.
In Windows 7 go to the Network and Sharing centre, set up new connection or network, select your router and fill in the details.
You can then dial that connection from the start menu. You can create as many connections as you like or just use one and change the username/password when you want to use another account.