SQL Server on Windows 7 RC and Windows Server 2008 R2 RC
All editions of SQL Server 2005 SP3 and SQL Server 2008 SP1 are supported on Windows 7 and Windows Server 2008 R2.
For SQL Server 2005, you need to be on SP3 to be compatible on Windows 7/WS08 R2
For SQL Server 2008, you need to be on SP1 to be compatible on Windows 7/WS08 R2
For more information on SQL Server Express and Windows 7 RC, please refer to SQL Server Express blog
For more information on Visual Studio and SQL Server Express on Windows 7 RC, please refer to this post.
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