Is there a special reason for using user instances?
If you cannot avoid using them, make sure the "user instance enabled" option is set for the SQL Server Express instance (look it up in Books Online).
If there aren't any rational reasons for using user instances, simply don't use them (i.e. don't use the "user instance" argument in the connection string or don't set it to true).
ML
---
Matija Lah, SQL Server MVP
http://milambda.blogspot.com