Loss

Hi, I have a situation with my installation. I set up a Radius server on a switch cisco catalyst 3750 with an AD (for now only one) using 802.1x with dynamic vlan assignment. The problem is that when the supplicant boots, he cannot enter his credentials, he's directly asked to log on the AD... without any network connection... Hard ... So I'm trying to force xp to prompt the user before the logon. Can you help me ?