create procedure dbo.Lookup (
    @UserName varchar(32), -- Or what ever you are using
    @Password varchar(32), -- Or whatever you are using
    @IsValid tinyint OUTPUT
)
as
begin
    select
        @IsValid = case when @Password = pw then 1 else 0 end
    from
        dbo.MyLoginTable
    where
        Username = @Username;
    return;
end