I had the same problem.
Uninstalled everything, that smelled like "2005": Devstudio,
Dotnet-Framework, SQL-Server, Document explorer, MSDN. Then I manually
deleted registry entries named visualstudio 8.0, sql-server, msdn 8.0.
Deleted also files in "Program files" for those products. Draped
everything with reboots.
It was a stupit job, but finaly it worked. Don't ask why.
Greeings Harald