The instructions at this website are provided by New Atlanta Communications.
In addition to these instructions, Esri provides its own steps for installation and configuration of ServletExec and ArcIMS.
For ServletExec 5.x ISAPI on Windows 2003 (IIS 6) and ArcIMS 9.0 click here.
For ServletExec 5.x ISAPI on Windows with IIS 4 or IIS 5 and ArcIMS 9.0 click here.
A listing of all such Esri knowledge base articles can be found here.