During my deployment of Exchange 2007 SP2 and also in the past when attempting to install the unified messaging role on server 2008, I encountered an issue
in that the UM services would not start. In event viewer, you will be presented with the following error in the application log.
Event Source: Microsoft Exchange Speech Engine
Event ID: 26191
Description: The main service was unable to start. Please check that it is configured correctly. Details: ‘Invalid class ‘.
Fun! After scouring the internet again for a resolution to this error and not finding anything I started to dig into the service and dependencies. The speech engine
service relies on both the RPC and WMI services to be running to start successfully. I check both and they were in fact running. After taking a step back to think
about the requirements for Exchange and many other Microsoft products I landed on the .Net Framework. Then it clicked, DISABLE ROLLBACK.
Without realizing, I made two changes at the same time so I am not sure which resolved the issue but, after changing the disable rollback registry setting and disabling
antivirus on the server, I was able to successfully run the SP2 install and start both services. The disable rollback registry key is located at,
Modify the setting from a 1 to a 0 and rerun setup. I can only assume that this was my problem as I did NOT have to unload antivirus on any other server role that was
upgraded to Exchange 2007 SP2. Hopefully this will help a frustrated Exchange admin…