Cybertof,
The benefits of IIS on 2003 over 2000 are not related to ASP.NET, but
you are provided with improved reliability. Before, on 2000 (IIS 5 I
believe), the IIS service handled the incoming requests. If your
application caused a fault for some reason, then the service would be
brought down. Now, the request handling mechanism (and much of the other
logic that is common to all applications) is handled in a kernel-level
driver. This way, your application can't bring down the site (although your
specific processing won't occur). This allows IIS to try and re-start the
application in the case of a fault, in order to provide a more seamless
experience in the event of a failure.
Hope this helps.