I'm just an average user of CC but if you're not sure, run System Restore (on XP) to create a System Restore point before you run CCleaner just to be on the safe side. What it cleans may vary from user to user, depending on what applications and peripherals you have.
The 2 main functions in CCleaner are 1)Cleaner and 2)Registry. Also in the Windows the Application tabs you'll have the choice of what to keep & what to ditch. Do you visit websites that keep your log name and password memorized for you? Then you might want to uncheck 'Cookies' for now so you won't have to hunt down names & passwords later. In Cleaner you can run "Analyze" and take a gander at what it's gathered for cleaning. Nothing is removed yet. Keep in mind there'll most likely be a ton of temporary internet cache composing the bulk of it if you don't currently have your browser running at the same time you run CC. (That's safe to give the ol' heave ho.) But look at the rest of it. See if you see any names of apps that you don't want to tamper with and uncheck them from the Windows or Application tabs as well, if they're listed. When you're satisfied press "Run Cleaner".
The deeper app is Registry. I've never had a problem with this but maybe that's just me. From what I've seen from other reg cleaners this one is the safest. Run "Scan For Issues", look at what it targeted and what dysfunctional category they're listed under. Nothing is removed yet. Uncheck any things you see that make you a bit leery and run "Fix Selected Issues". You did remember to run System Restore earlier didn't you? Good. You're right as rain. Have a cookie.
No, I'm not an Admin or a Moderator, but I did stay at a Holiday Inn Express.
Oh, and yes it will take you a fair amount of time to learn all of CCs' little goodies, but not a humungous amount.
If your PC is running just fine you don't 'need' to do a registry scan, but we do because all of our names are Felix Unger and we hate dirt.