I think you will find that ATi tool changes the way CCC does it's business and therefore causes CCC to disable overclocking. If I am right I think you will find that there is a way to overcome this, the first thing to do is to not have ATI tool starting at startup so uncheck that in the settings in ATi tool, you can always just open ATi tool once into your desktop and choose your profile then tweak to your hearts content.
You then need to go to "START" at the bottom left of your screen, then click on "run" and type in the command "msconfig", you will then get a configuration screen, go to the "services" tab and you should find one or two obviously CCC/ATi related proggies disabled/unchecked (these are the ones ATi tool may have played with),also at this time just check the startup tag to ensure that the ATi/CCC entries (if any) are checked to startup, excluding of course ATi tool as you will just be undoing what you have just done enable them and apply and quit, you will be asked to restart your PC, make sure you dont have ATI tool starting at startup as previously mentioned and you should find that once restarted if you go into CCC (you should have that set to starting up with windows now if not before) you should have the overdrive option.
Now all I have said here depends on your version of windows and ATi tool but give it a go, the good thing about this is you can painlessly reverse it if it didnt work.
