In XP , right click on the lan connection icon in the system tray, and go into the properties.
There in TCP/IP settings see if you are using DHCP( ie determine IP address automatically) or you have static IP address, gateway and dns settings entered.
If its dhcp, just enable the dhcp daemon in ubuntu( refer www.ubuntuforums.org for that) in network settings menu in ubuntu and you are done.
Else better way is to setup the router in bridge mode to enable pppoe( its described somewhere in the MTNL Triband forum) and run "sudo pppoeconf" at the terminal to setup pppoe dialing.
There in TCP/IP settings see if you are using DHCP( ie determine IP address automatically) or you have static IP address, gateway and dns settings entered.
If its dhcp, just enable the dhcp daemon in ubuntu( refer www.ubuntuforums.org for that) in network settings menu in ubuntu and you are done.
Else better way is to setup the router in bridge mode to enable pppoe( its described somewhere in the MTNL Triband forum) and run "sudo pppoeconf" at the terminal to setup pppoe dialing.