It will be to do with dns and as kkob put it add an entry to your hosts file. When you type in a web url it wants to change that name to an ip address and that is what is not happening because the dns servers both of you are using are different. You could try changing the dns information on your computer or your router. Opendns have a page showing you how to do it. For xp it is here.
I've never been blocked or used them but years ago I had a problem with BT dns servers and I used 22.214.171.124 & 126.96.36.199 and they worked fine. Use the link above to put in the addresses i just gave you and it should work ok.