The SQL Writer Service provides added functionality for backup and restore of SQL Server 2005 through the Volume Shadow Copy Service framework.

This SQL Writer Service is automatically installed but not enabled by default. It must be explicitly enabled to run on the server machine and must be running at the time that the Volume Shadow Copy Service (VSS) application requests a backup or restore. Use the Microsoft Windows Services applet to configure the service. The SQL Writer Service installs on all operating systems but is only needed on server systems. For Microsoft Windows XP, use the MSDE Writer.

SQL Server 2005 Service Pack 2 configures SQL Writer to start automatically.

http://msdn.microsoft.com/en-us/library/ms175536(SQL.90).aspx