First, you can uninstall and reinstall an unlimited number of times without any activation problems, as long as you don't reinstall Windows from scratch or change hardware. The Uninstaller will never remove your registration info, in any case.
Running the Uninstaller the Reinstalling should normally work, unless there are other unusual permission settings that are in effect on your machine.
In this case, it might be useful to try these:
- running FS9 as Administrator, at least once to activate the scenery.
- turning off Vista UAC ( look here to see how to do this ) http://www.petri.co.il/disable_uac_in_windows_vista.htm