20 min to log on

I have an account with a school .All the computers when logging on to domain take 20 min to load user preferences. Once logged on everything is fine. Now I checked all the hardware and it is fine. The computer that is right next to the server logs on fine connected another computer that was taking a long time to log on to same Ethernet cable and it did same thing .So it has to be something with active directory right? Please help I just can?t figure this one out.