When installing Enterprise Library 4.1 today I recived the “This version of Enterprise Library cannot be installed side-by-side with version 4.0. Please uninstall …” message even though I had uninstalled version 4.0 already (and re-booted). After searching for the problem, I quickly found som tips (for example this) stating that removing the registry key “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Enterprise Library v4” would solve the problem, but I did not have a key like that. It turns out though that the key is located here “HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Enterprise Library v4” on a 64-bit versions of Vista. After removing that key, version 4.1 installed just fine.