Forum Moderators: phranque
You can't do it and you really have no choice but to move to a competent hosting situation.
[edited by: Marcia at 7:43 am (utc) on May 20, 2007]
The two most obvious possibilities:
1. Your DNS record for "www" is pointed to the wrong IP address.
2. You are using a shared-IP hosting account, and the server is mis-configured. Somebody (either your host or yourself) has failed to created a virtual host for your site.
In either case, if your host can't figure it out - I'll echo the advice of others to get another host.