I didn't have to do that, when reinstalling.. The only UAC issues I've had with ATi Tool are when you actually run the program, if you don't run it as an Administrator when User Account Control is switched on. The program won't load correctly, and all the options will be some what locked off or they just don't appear.
Check that box when UAC is switched on, to run the program as an Administrator by default. You may experience similar problems with nTune so, do the same for nTune.
Btw, if you do have nTune with your OC'in options locked off as they are these days on Vista, ggrrr.. Once you've overclocked using ATi Tool, the OC'in options will be active in nTune and you'll be able to OC using nTune.