Agent won't install on a server which previously had an older version agent installed. Error: "Er (4270809)

When trying to install a new agent on a server which previously had an older version ChangeAuditor Agent, the install halts with the following:
"Setup failed to make a backup copy of the local agent database. Some events may be lost. Continue?"

When you click "OK" you get the following:
"Error1714. The older version of Quest ChangeAuditor Agent 5 (x64) cannot be removed. Contact your technical support group"

Uninstalling the agent, removing the ChangeAuditor keys in HKLM/Software/Quest Software, and deleting all files at C:\Program Files\Quest Software\Agent then rebooting the server before redeploying an agent does not resolve the issue.

There may be left over keys from the Agent database.

Run the MSI installer with verbose logging enabled:
msiexec /i "<system drive>\Quest ChangeAuditor Agent 5 (x64).msi" /L*V "C:\ChangeAuditor_Agent.log"

The logging should show you the exact key which needs removed. It may be something like:
"MSI (s) (AC:38) [11:07:21:444]: Unexpected or missing value (name: 'PackageName', value: '') in key 'HKLM\Software\Classes\Installer\Products\541CFF833270B1C479ECB5021AEAFFE2\SourceList'"

Deleting this key should allow installation of the new agent.