xp slow log after altering dhcp server

up to recently my servcer (Win2000)was the dhcp server. We installed a broadband router and this now acts as the dhcp server. However since the change the xp pro machines are taking ages to log on, i also have win98 computers on the network and these work perfectly. hope the discription is ok as i dont regard myself as and expert. I teach in secondary school and am trying to maintain our school network.