"Generating user instances in SQL Server is disabled"

I don't know why, but starting about two weeks ago SQL server is giving me error messages. They arise the first time I try to log in to a web application, one which I'm developing under Visual Web Developer 2005 Express Edition. The error message:

Generating user instances in SQL Server is disabled. Use sp_configure 'user instances enabled' to generate user instances. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.Data.SqlClient.SqlException: Generating user instances in SQL Server is disabled. Use sp_configure 'user instances enabled' to generate user instances.

This problem recurs every time I restart my Windows XP VM (running under VMware Fusion). Until I figure out why the problem recurs, here's the SQL which makes it go away:

exec sp_configure 'user instances enabled', 1;
reconfigure;
go