You can install Outlook Web Access (OWA) files during the initial Exchange setup, or afterwards. Exchange can be installed on the same server as IIS, or it can be installed on a different server that can be accessed by the server where OWA is installed. When installing OWA and Exchange on the same server, the World Wide Web Publishing service must be installed and running.
It sounds like you are trying to install OWA on your Exchange server, and you have not yet installed IIS or Active Server Pages. You might be receiving the error documented in Microsoft Knowledge Base article Q173509
Since you do have IIS installed on your other server, you could install Exchange and OWA on that server and your users can access the server from there. If you end up installing OWA on your existing Exchange server, don't forget to re-apply the Exchange Service Pack afterwards.
For more information on installing and using OWA, check out the Exchange 5.5 product documentation.
Hope this helps!
This was first published in February 2001