Cannot use the reserved user or role name db_datareader.

Hi, I have got a problem. When I try to access my database table Users, I get the following error:
SELECT permission denied on object 'Users', database 'Users', owner 'dbo'.
SoI tried to grand this select command in MS Web Data Administration, butit doesnt work. When I try to grand db_datareader role to dbo, I getthe following error
[Microsoft][ODBC SQL Server Driver][SQL Server]Cannot use the reserved user or role name 'db_datareader'.

Does someone have an idea where could be a problem?

You don't need to be granting db_datareader to dbo, you need to beputting the user that you're accessing the database with in that role,or at least giving it SELECT permissions on that table, or better yet,giving it EXECUTE permissions on a stored proc that selects from thattable.