Wait. Are you saying now that your failure happens during uninstall? Because so far my assumption was you get failure when installing new version at the point when services have to be started for new binaries. Actually, that is what you said, but I missed it.
You can run installer in debug mode (run installer from version you have currently installed) to get more information (sometimes installer requires same version, but I have seen it only with some very very old installations). Debug should give you details what is wrong. Last resort, and not so difficult, is to stop services (maybe they are already - no data on that), remove binaries and remove registry entries. That is manual uninstall to say so. Of course, you should make sure that in all cases you have proper admin rights.