if the hostname is missing you can just create the key and cycle services. if you need to reinstall follow the steps below.
I am including the instructions for a manual uninstall of CommVault from any Client Machine. Please let me know if this agent is also a Media Agent or an application such as CDR or File Archiver that has a kernel level driver as this would be a similar procedure but there are some additional things to look out for. This process is quick and relatively painless and will accomplish the same task as the Add/Remove from the Control Panel.
1) Stop the CommVault Services on the Client
2) Navigate to the installation drive under Program Files remove the CommVault and/or CommVault Systems Folder
3) Export the registry
4) Open the registry and delete the following keys:
Please note in versions earlier than 7.0, Instancexxx would be ControlSetxxx
5) Once the registry keys and the installation folder are removed you can then reinstall the agent using the same client and host name.