I'm not using Debian (now don't all start yelling) but I had a combo dsl/router (cayman) like this.
If you have internet connectivity in windows (can ping www.yahoo.com
in a dos box) then try the next stage.
I'm almost certain this 2wire box will expect you to be a DHCP client.
see if dhcpd is running
ps -ef |grep dhcpd
and you should get something like
root 1046 1 0 11:34 ? 00:00:00 /usr/sbin/dhcpd eth0
Try starting it manually.
If not, then find the Debian dhcpd package, install it and run manually.
then try pinging www.yahoo.com
If none of this hackery is successful, then back to windows and see if you can get into the 2wire box configs
see if its set up to expect dhcp clients connecting on its LAN side.
To get some insight as to how to set this stuff up, take a look at the (rather dated, alas)
While you are there, take a look at the networking howtos. I know it may be a bit overwhelming, but regard it as a tribute to all the unsung heroes who have contributed so much to the Linux community.