Have you like me had issues uninstalling an earlier build of Exchange Server 2007 (both via Add/Remove Programs as well as ExSetup.exe /roles:installed roles /mode:uninstall), before applying a newer one? Then try using below command:
MsiExec.exe /X{24B2C164-DE66-44FE-B468-A46D9D5E6B31}
This has saved me from switching to a new virtual guest several times already.