Also note that Apache 1.3.26 on Windows is also supported if you use ServletExec 4.2 or the ServletExec 4.1.1 installer that is found on the ArcIMS 4.0.1 CD (ServletExec_AS_411a.exe). That particular 4.1.1 installer is not available on the New Atlanta Website. It is a special version of the SE 4.1.1 installer that was created especially for Esri, which looks for the Apache 1.3.26 registry entries in addition to the earlier versions listed above.
Instructions for integrating other versions of ServletExec and ArcIMS can be found here.
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 4.1.1 AS for Apache on Windows and ArcIMS 4.0.1 click here.
A listing of all such Esri knowledge base articles can be found here.