Hello,
You can try to look at the settings from Windows (i don't know how to get there by heart, but someone here does???), and try to recreate those settings in Ubuntu. I first had problems too with my modem/router (speedtouch), but after putting exactly the same config in Linux as in Windows, I could connect after a reboot of both my pc and my router.
If you can't get DHCP to work, why not try and use a static ip address? Make sure you know where to find your router on the network (it's ip). Type "ifconfig" in the terminal (without quotes) so you can see what the address of your standard gateway (=router) is. If you don't see anything mentioned about gateways there, try opening a shell under windows (Win key+R and type cmd in the window that appears) and type "ipconfig" (without quotes) there. Note that under linux, you should type ifconfig with an F, whereas on Windows you have to type ipconfig with a P.
I hope this helps!
- SeySayux