These instructions apply only to ServletExec 4.1.1 or 4.2 with ArcIMS 4.0.1, using Apache web server (v. 1.3.12, 1.3.14, 1.3.19, or 1.3.20) on the Windows operating system.
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.

 

Detailed installation information (including screenshots) can be found in the ArcIMS 4.0.1 Installation Guide found on the ArcIMS 4.0.1 Installation CD. The specific file on the CD is /Documentation/Install.htm. The Introduction chapter contains a section entitled "5 steps to getting ArcIMS up and running" which gives a good overview.
The Microsoft Windows chapter gives more specific details.
Note: Section 3a will definately be of particular interest at this step, as it walks you through every aspect of installing ArcIMS 4.0.1