well,
I'm inclined to say it's a problem with your ISP's nameservers
but windows doesn't have any issues

if you're using DHCP (and not a static IP)
your router should pass on your ISP's DNS info to ubuntu

quick fix,
dig through your routers setup
you should be able to find the primary & secondary DNS addresses used by your ISP
(probably in some sort of status page)
note these down

then,
Code:
sudo network-admin
under DNS,
delete your router's IP
Add the 2 nameservers you noted down