An error message similar to the following is displayed when attempting to add additional features by running the setup.exe from the original installation CD after having installed a service pack for ArcIMS:
"ArcGIS ArcIMS Version 9.1.750 is already installed. You must uninstall the existing version before installing ArcGIS ArcIMS Version 9.1.722. Do you want to uninstall the existing version of ArcGIS ArcIMS"
When a service pack is installed, the build number of ArcIMS is updated in the Windows Registry. The MSI installer checks this build number against the original CD as they need to match in order to add additional features.
Depending on what additional components are added, reapply the service pack. Check the documentation for the service pack to see what files were updated.