Found it.
Q. What release of SQL Server 2005 will run on Windows Vista?
A. SQL Server 2005 Express Edition with Service Pack 1 will run on Windows Vista but has known issues with User Access Control. For all other editions of SQL Server 2005, Service Pack 2 will be required for Windows Vista and Windows Server "Longhorn" support when those products are released.
http://www.microsoft.com/sql/prodinfo/previousversions/faq-sql_on_vista.mspx