I was trying to install latest version of the Enterprise library (4.1) and I was constantly receiving this error.
My previous version of the Enterprise Library was uninstalled (4.0) so I did not have any idea what it went wrong. On the Patterns and Practices codeplex page under discussion, I found resolution to this problem.
Remove the key “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Enterprise Library v4“.
The issue with the MSIs for 3.1 and 4.0 is that there are some duplicate component IDs. When uninstalling, reference counting kicks in and some components, like this registry key, are not deleted and must be manually cleared.