See IIS Informant articles. There are three, count 'em three ways to create multiple websites on IIS 4 and 5.
First way is to use an unique IP for each site.
The second way allows you to create multiple IP's on one address by designating a non-standard TCP port number for the designated web to use. This is the way the Administrative website winds up on the same installation as the default website and both can be accessed. However, you have to designate the unique port number in the URL when accessing it. Thus we come the next best thing, host headers. The good news is this is relatively easy to do in IIS and it works very well.