Uninstallation of incorrectly installed package / RU

Sometimes it happens, that it is not possible to install some updates for example interim update for Exchange (RU), because previous installation was stuck.
The normal way of installing RU works like this:

1. Disable Forefront if installed (FSCUTILITY /DISABLE) – nice article for example here: http://www.urtech.ca/2012/03/solved-how-to-disable-forefront-for-exchange-without-killing-exchange/
2. Run installation of new RU (This will automatically uninstall prevous RU and install new one)
3. Reboot the server
4. Enable Forefront if installed (FSCUTILITY /ENABLE)

In our case the previous try recorded errors in event log:Event ID 1603 or Event ID 2771.
Both errors mean, that there was a problem uninstalling previous version of RU.

We have tried to:

1. uninstall RU manually but without success:
a/ selected your favorite way to uninstall RU (whether to use Control Panel -> Programs and features -> Select view installed updates -> Locate and uninstall needed package or using command line msiexec.exe /x <packagename.msp>
b/ Reboot the server

2. Install previous RU again
No success, the package cannot be applied to the version of the product that is installed on this computer (In normal words the update is already installed)

3. Install new RU

4. Cause 1)
Installation of previous RU went incorrect and package is in half installed state
we have tried to repair installation of the package by executing msiexec.exe /fa <path and name of RU package.msp>, but we have got the same errors (Event ID 1602)

5. Cause 2)
Previous RU package have been removed incorrectly and registry entries of this package are still visible in Programs and features TAB.
This was our case. We have done uninstallation of previous RU update for Exchange 2010 SP2, however record that RU is installed was still present in registry database of installed packages.
Database is located in the following registry key:


and each particular subkey means one installed package. It is in most cases presented as package GUID. If you click on some subkey you will see the similar result as shown in the picture. The important stuff is package name and uninstallation string.
We copied uninstallation string and run it in elevated command prompt but it failed again.


We made sure that previous RU was really uninstalled by checking Exchange version in Powershell, however record was still in the key mentioned above. We have exported the particular registry key and deleted it. After that we were able to install the same RU again and continue.


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