problem in security permission for mscorlib

Question

this is the error i am getting Description: The application attempted to perform an operation not allowed by the security policy. To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file. Exception Details: System.Security.SecurityException: Request for the permission of type 'System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed. i tried to change the trust level in web.config file but it is not allowing me to do that also it says something about tag where allowOverride is set to false in some internal file can any one tell me where to chane this to true