SOLVED: The SQL Server 2005 Express Tools are installed. To continue, remove the SQL Server 2005 Express Tools.
When installing SQL Server 2008 or the SQL Server 2008 Management Tools I received the error:
The SQL Server 2005 Express Tools are installed. To continue, remove the SQL Server 2005 Express Tools.
Some searching uncovered this thread which someone recommended deleting the registry key:
HKLM\SOFTWARE\Microsoft\Microsoft SQL Server\90
This is a bit drastic so I experimented and found that I could simply delete the key:
32 bit: HKLM\SOFTWARE\Microsoft\Microsoft SQL Server\90\Tools
64 bit: HKLM\SOFTWARE\Wow6432Node\Microsoft\Microsoft SQL Server\90\Tools
I did not even need to restart the install, just click re-run on the rules page and the prerequisite rules all passed this time.
