LowRider2112
These steps almost always provide some insight:
1. Verify Internet Explorer's security zone settings on the client with error. See if your control works in the Trusted Sites list for example, or set a particular zone to Low. If it starts working on lower settings, you know then IE is deciding your control isn't "safe", like for example it isn't code signed, or you haven't implemented IObjectSafety, or something is wrong with the user permissions (yes, it can still happen with Admin users).
2. You can use the Code Download Log Viewer utility to view what the Code Downloader is saying about your control's installation. You can find that utility here:
http://msdn.microsoft.com/archive/default.asp url=/archive/en-us/samples/internet/browsertools/cdllogvw/default.asp
3. Use the DEPENDS.EXE utility (comes with Visual Studio) to verify what libraries your control, heh, "depends" on. Maybe there is a .dll that it needs that doesn't exist on some of the client machines. You need to run DEPENDS on the client computer that doesn't work with a copy of your control manually copied to the client machine. Another hint that it is this issue is if you manually try to "regsvr32" your control on the client machine and it fails.
Here's a reference to Microsoft's KB article on what to do:
http://support.microsoft.com/kb/252937