dbreader and dbwriter

If you have execution permission error when you run your ASP.NET, assigning dbreader and dbwriter roles to the ASPNET user will not help, especially your application only uses stored procedures. dbreader and dbwriter serve no purpose for this. They are related to table access. In this case, you need the following statement (if you're using SQL 2005).

GRANT EXECUTE ON SCHEMEA::dbo TO ASPNET