I had a strange error trying to install SQL 2008. It complained that there are SQL 2005 Express Tools installed. Even after uninstalling all traces of anything that has to do with SQL 2005, the error was still the same.
As per article below, I found registry keys in
HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SQL Server\90\Tools\ShellSEM
Sql2005SsmsExpressFacet - How to continue your SQL 2008 installation - Administration - SQL Server Nation
The reason was SSMSToolsPack that I installed some time ago and (probably) never used. Uninstalling that made the SQL 2008 installation proceed.