I was pushing out flash updates using SCCM 2007 and the exit code on a few computers was 1603, failure non retry! argh.
After testing the installer locally on the problematic computer, no amount of uninstallating/reinstalling would solve the issue. I finally succeeded with the update. After uninstalling all flash player items, you need to find and remove all flash related registry items here:
[-HKEY_CLASSES_ROOT\Installer\Products\
You'll need to go down them individually (or search) and delete them. Once that is done close all browser windows and launch the installers again. It should succeed this time.