For testing purposes or any reasons you may need to install an older version of the WSSA after uninstalling a later version.
In certain case the WSSA will refuse to install indicating an newer version of the product is already present.
The MSI based uninstallation process may have failed to remove product entries on the Registry user hive (HKCU\Installer\Products\<guid>)
You will need to clean up the Windows registry of references from the previous (newer) WSS installation.
Important Note: this should be limited to the "Installer\Products" keys under HKCU or HKLM. Per Microsoft documentation, care must be taken when making deletions in the registry.
Once the registry is clean of the later version data the MSI installation should complete without any issues.
In order to gather some additional information do help with debugging installation of WSS Agent on Windows, you can run with additional logging:
c:\path\to\installer.msi /L*V \path\to\output.log
After running, gather \path\to\output.log and check for errors in the log file.