I had an issue where an application was erroring every time I tried to open it.  I tried to uninstall the application, but this failed as the source files were no longer available.  I tried to install the latest version of the app, but this failed because an older version was still installed!  Having downloaded many of the free uninstall utilities and got no where I decided a brute force approach was worth trying.  Searching through the registry I found the entry for the application:

HKEY_CURRENT_USER\Software\Microsoft\Installer\Products\10EAF4144327A104B8D4CB44666B4DCE

I wanted to trick the new version of the app so it couldn’t tell I had an older version already installed.  I renamed the above registry key to:

HKEY_CURRENT_USER\Software\Microsoft\Installer\Products\10EAF4144327A104B8D4CB44666B4DCE-OLD

The new version of the application installed perfectly and has been working ever since.  This technique may not work with all applications and should be a last resort, but it worked for me.