You can also try to manually uninstall following the steps below:
1) Stop the Services on the Client
2) Navigate to the installation drive under Program Files remove the CommVault and/or CommVault Systems Folder
(You may receive a read only error which would mean the client would need to be rebooted prior to the deletion.)
3) Backup the registry
4) Open the registry and delete the following keys:
please not in versions earlier than 7.0, Instancexxx would be ControlSetxxx
5) Once the registry keys and the installation folder are removed you can then deconfigure the client in the GUI to reclaim the license that is in use or reinstall the agent using the same client and host name.