Avamar SQL/Exchange agent remove when the old registry still exists after uninstallation

Remove any legacy Avamar SQL/Exchange Agent left on the client(other than the Backup Agent service).

>> Uninstall the old Avamar Agent plugin by using the below command in the administrator command prompt:

# msiexec /x <AvamarClient-sql-plugin-package-ver-.msi> /L*v logxxx.txt /norestart

>> After the uninstallation, try to install the new Avamar Agent plugin.

>> If the Avamar Agent Plugin Installation Wizard prompts that the Agent Plugin is already installed and is still reflection the older version. Then perform the below steps:

>> As administrator (PowerShell) runs this command:

# Get-wmiobject Win32_Product | Format-Table IdentifyingNumber, Name

>> Check for any Avamar Plugin Agent related names or version number in the output. 

>> Use cmd to delete it:

# msiexec /x <IdentifyingNumber>

>> Then use "Microsoft Software removal tool" - Cleanup tool to verify if there is still any existing residue and registries of the old plugin.
https://support.microsoft.com/en-us/help/17588/fix-problems-that-block-programs-from-being-installed-or-removed

>> Then reboot the client and retry installing the Avamar SQL/Exchange Agent Plugin again.