If you're having trouble installing the Symantec Endpoint 11 client on a machine that was previously running Symantec AntiVirus server components you may have have to clean up the Windows Installer entries.

One of my servers still had an old AMS entry in the Add/Remove Programs control panel, even though the product files were completely uninstalled. This caused the SEP11 client to back out part way through the installation without any specific errors. Removing the AMS entry using Microsoft's Windows Installer Cleanup Utility [1] fixed the problem.