IIS 5.0 question

We have two main domains that are routed to the same web server and to same IP. My boss wants the default URL to always appear the same, i.e. when a user types www.xxx.com, he wants the URL in the address window to be automatically rewritten as www.yyy.com, instead. Both domains of course are valid and point to same default page. Can’t figure out how to do that, just yet. I’ll appreciate anyone’s help on this one.
Thanks!
Robert