Installing WEB server under win2k

I am connecting through cable modem-I have dynamic IP but it doesn’t change.I bought domain name & point it to my IP and server where I installed DNS & IIS. this server is doing Internet sharing to my second computer so it has one dynamic IP & static virtual IP. I succeed to enter my site by writing http://. but not http://www.. My IIS seem to work fine & I think my DNS too. what do I need to do?