Hi there. I d/l CCleaner and used it for the first time today. It's a great bit of kit....BUT.
I have an immense amount of crap that starts at startup (4OD, BBC iPlayer, realtime, msn, skype, etc) so the tool which allowed you to select which programs can start at startup is really useful for me. Unfortunately, I kinda went a little crazy and now none of the media buttons on my keyboard work (Play, skip track etc directly on keyboard, without having to be in the media player window).
Is there any way that I can undo the changes I made?
You could try a System Restore to a date before you "changed" things.
If you're using XP, Start > Control Panel > Performance and Maintenance > In the left menu, "System Restore".
And possibly use something like "What's Running", startup menu section in future, to simply check and uncheck stuff in your Startup. That way you can put things back again.
Unfortunately you can't reverse CC's actions (apart from reinstating a registry backup). I am not familiar with the software you describe as your startup crap, so I can't give specific advice, but my money goes on you having to set up your keyboard hotkeys again. Perhaps if you restart the media players you disabled, or the hotkey poller? Good luck!