A fix for that annonying per-user instance problem with SQLExpress
Thanks to fellow DevelopMentor "Brain" Kevin Jones (RSS) for pointing this one out.
These problems left me stranded for days, then today, on another machine, I hit a different set of problems I kept getting the following "Failed to generate user instance of SQL Server due to a failure in starting the process for the user instance. The connection will be closed." and if I set "User Instance" to false the following "An attempt to attach an auto-named database for file C:\[app_path]\ASPNetDB.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share."
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If you don't want to follow the link, the short answer is that SQLExpress creates a directory per user in "c:\Documents and Settings\[user]\Local Settings\Application Data\Microsoft\Microsoft SQL Server Data\SQLEXPRESS" that it uses to store information. Deleting this directory has fixed both of my problems.