I think you are facing Orphan Users Problem
1. You had to generate the logins script on SQL Server 2005 with Sp_HelpRevLogin.
2. Once generated you had to run that script on SQL Server 2008.
This process creates the logins with the same SIDs on the new server so that you don't face this problem on your new server. If you havn't executed this process in above said manner then you are most likely to face this.
In that case, SP_Change_Users_Login
OR ALTER LOGIN
are your options. You can do any of following:
Execute SP_Change_Users_Login 'Update_One','UserName','LoginName'
ALTER USER UserName WITH LOGIN = LoginName
You can read more here