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If you encounter error such as "Object reference not set to an instance of an object" during installation or uninstallation, the most probable cause would be you have installed earlier version of SQL Server Express (e.g. 2000, 2005, etc). To solve the problem, uninstall earlier version of the SQL Server Expression version, then install the SQL Server Express 2008. If you need those earlier version of SQL Server Expression, you can install them after you install SQL Server Express 2008.