Dê uma chance:
To work around this problem, follow these steps:
Start Registry Editor, and then locate one of the following registry subkeys:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.2005\Setup
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.X\Setup
Note Only one of these registry subkeys will exist on your computer.
Empty the following registry entries:
SQLGroup
FTSGroup
AGTGroup
Exit Registry Editor.
Uninstall SQL Server 2005 or install SQL Server 2005 SP1.
Menciona que é para o SP1, mas vale a pena tentar. Trecho retirado do link