[0xC3EC7823] The specified Office Communications Server 2007 server is not installed on this machine.

Tried installing OCS Standard, but failing spectacularly every time.

This didn’t make any sense to me for a while, since the components were installed, but the installer refused to activate the product. The MMC didn’t show that services were installed either.

How I resolved this was by uninstalling each OCS component from the add/remove programs in the Control Panel, and running iisreset on the command line. 

After this OCS installed cleanly.