It appears some SQL Server 2005 ghosts are still hiding somewhere in your system.
To install Denali, you will need to remove the following keys from the registry:

HKLM\SOFTWARE\Microsoft\Microsoft SQL Server\90
HKLM\SOFTWARE\Wow6432Node\Microsoft\Microsoft SQL Server\90

Once removed, just Re-Run the checks, now it should succeed.
Note that you do not need to delete corresponding folders on your disk.

This article gets a credit for pointing in the right direction.