Need to determine if this problem is related to this issue:
http://www.opcfoundation.org/forum/viewtopic.php?p=9622#9622

This problem is not related to the issue with UAC on Vista. I found this problem when I have been running my own Wix-based setup on XP (proxy/stub modules have not been registered).

I found the workaround for Wix: manually include actions SelfUnregModules and SelfRegModules in the InstallExecuteSequence section:
<InstallExecuteSequence>
<SelfUnregModules />
<SelfRegModules />
...
</InstallExecuteSequence>

About UAC and issue 0000549...
Currently user need to have Administrator privileges for run the setup without errors on Vista (UAC is enabled) (because opcenum.exe can not be registered).

Maybe the workaround for this problem: set 'requireAdministrator' for 'UAC Execution Level' in the embedded manifest for opcenum.exe (in VS2008 Project-Properties-Configuration Properties-Linker-Manifest File). After that for each run openum.exe the Vista will display Administrator privileges request dialog.

Please see attached log file at line 2407 (error from issue 0000549).

The merge modules have worked for years as is and it is not clear why the manual change to the MSM is required.

"Some tools, such as the Windows Installer XML (WiX) toolset, may only generate standard actions in their execute sequences if the corresponding feature has been used. You should list all the standard actions that are required to use your module - for example, if you use the Class table, ensure that RegisterClassInfo and UnregisterClassInfo are listed in your ModuleInstallExecuteSequence (and RegisterClassInfo is in the ModuleAdvtExecuteSequence)."

http://msdn.microsoft.com/en-us/library/aa370049(VS.85).aspx

It appears that this ia a bug in Visual Studio 2005 Installer tool and cannot be resolved without using a different Installer tool to build the MSM.

An upgrade to VS2008 may solve the problem.

An upgrade to Wix may solve the problem ;).

Upgraded projects to VS2008