Changing hard disk

My hard disk needs to be changed since it is damaged

How do i continue to use CCleaner on the new hard disk without paying again?

Simply install CCleaner Free on the new drive and open it.

In CCleaner Free go to Options>About>Upgrade to Pro and enter you registered key and name when asked, then click on 'Register'.

More info and screenshots: https://support.piriform.com/hc/en-us/articles/204043844-How-to-register-and-activate-CCleaner-Professional

If you don't know your key, and can't get it from CCleaner Options>About>Licence Information on the old drive, then you can use this to get it sent to you again:

https://www.ccleaner.com/support/license-lookup