If you have never used CCleaner before, please keep your comments to yourself. CCleaner has been around for a LONG time & AFAIK, it has NEVER been found to be harmful. CCleaner doesn't touch the registry unless the user elects to run the Registry scanner. Even then, it doesn't delete anything until the user gives the OK. It provides the option to pick & choice registry entries from a list & can also create a registry backup before deleting, so unless the user is a complete moron, he (or she) can easily cover his (or her) azz.
The Cleaner deletes temp files, temp history files, cookies, index.dat files, clipboard, memory dumps, etc. These files build up on the system & slow it down. The average user never touches the IE settings. The default in IE is to save all browsing history for 20 days plus the "disk space used" setting can be HUGE. With the default IE settings, several GB of useless garbage will accumulate & slow the system.
Personally, I have IE history set to zero days & disk space set to 50MB. And I run CCleaner at the end of every single browsing session. If I'm on/off the internet 5 times in a day, I will run CCleaner 5 times. Each time it will clear at least 100MB of garbage. The leaner you keep your system, the better it will run.