Just use the uninstall option on the client or use the retire option from Command Center which removes the client entirely, but I assume you did this already. Curious to what kind of issue you are experiencing. 

Hi,

after the uninstallation I'm trying to reinstall but I see there are leftover in the registry.

Hi @Jean-Paul ,

You can delete CommVault Systems in Registry and Windows Explorer.

You can check this Post also.

Best Regards,

Sebastien