IIS version 5.x, standard on Windows XP Professional and Windows 2000, only allow single website to be created.  There are articles that talks about the workaround to enable support for multiple sites but with the limitation of only one site being active at a given time.  The suggested solution is only appropriate in simple case of switching sites for development purposes, or any other non-production proposes.

Another option is to create an ASP or ASP.NET page to inspect incoming URLs and route them accordingly to a location under the same website.  This approach is not advisable as the adminstrative interfaces for main and sub-site contents overlap, with no real way to keep them synchronised.

The support for multiple sites on a single IIS server is crucial for a single server environment to support sub-domain(s) and multiple web sites (as opposed to applications).  For real support of multiple sites on a single IIS server instance, the options are limited to IIS version 6 and above.