You cannot use the ActiveX Installer service to install ActiveX controls in a standard user environment on a Windows Vista-based client computer, and event 4100 is logged
Consider the following scenario. You try to deploy some ActiveX controls on a network. In order to deploy and update these ActiveX controls in a standard user environment, you use the ActiveX Installer service on the client computers. However, you may be unable to install the ActiveX controls in a standard user environment. Additionally, an event that resembles the following is logged in the Application log:
Log Name: Application
Event ID: 4100
Description: Failed to download ActiveX control
Note: This issue occurs only when you use NTLM authentication between the client computer and the proxy server. The issue does not occur if you use Kerberos authentication.
In this scenario, you must log on to the computer as an administrator and then install the ActiveX controls without using the ActiveX Installer Service.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB951585.