The following steps should work fine on the Client machine.  This should be done when a Client has just the File System IDA installed.  If it has other components, it may be best to open a call with the Support Hotline:

Remove the agent from Control Panel Add/Remove Programs if it is present.

Then, Go to 
Start>Run> 
Type in Regedit>click OK (or just hit enter) 

Remember to make a backup of the registry before making any changes 

Navigate to 

\HKLM\Software\Commvault Systems\ 
delete the key. 

Next we have to go to the 
HKLM\System\Current ControlSet\Services\ 
delete the GxEvMgrC, the and the GxCVD keys. 

Lastly go to the CommVault Folder in the File system and delete that. (C:\Program Files\Commvault Systems or C:\Program Files\Commvault)

To Reinstall, Run Setup and the install should run fine