Cannot open user default database. Login failed.

[micky]

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Cannot open user default database. Login failed.
Login failed for user 'NT AUTHORITY\SYSTEM'.
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my website consist of three wesite in a solution.

Solution(http://localhost/solution/)
-App_Data/db.mdf
-a(http://localhost/solution/a/)
-b(http://localhost/solution/b/)
-c(http://localhost/solution/c/)

a,b,c sites are using db.mdf file in app_data folder of solution project
so i made connection string like below.

<add name="ConnectionString" connectionString="Data
Source=.\SQLExpress;Integrated Security=True;User
Instance=True;AttachDBFilename=C:\Inetpub\wwwroot\Solution\App_Data\db.mdf"
providerName="System.Data.SqlClient" />

the problem is ...
open a,b or c website and then i also open vs2005 to modify database
error occur to me...like Unc share...

in opposite , it will be same result ...

help me~~