To resolve this issue, please follow the following steps:
1. Start the SQL Server 2005 Management Studio Express.
2. Connect to the default instance of that server (Your Machine Name\SQLEXPRESS)
3. Then open a New Query Window. Please check that you are connecting to the master database
4. Type: exec sp_configure ‘user instances enabled’, 1
5. Type: RECONFIGURE
6. Restart the SQL Server
Hope, this will resolve this issue.