dot1x authentication on windows xp and windows 2000 workstations

we're trying to test dot1x authentication on wired LAN and the set-up was already completed according to the cisco documentation on setting up dot1x for wired network, we've already tried to acquire ip address thru authentication and it worked, the problem is everytime we try to restart the workstation it already has the same ip we've acquired before we restarted the workstation. how can we make the workstation prompt first for auuthentication before it acquire ip address upon startup???