port 135 dce

Hi i am having a problem i have 150 win 98 systems and 50 xp systems on a nt4 domain controller the problem is the ip address lease runs out every 3 days on the systems and today about 20 systems cant connect to dhcp to renew there ip address the thing i have noticed is on the xp systems port 135 is open dce?i am sure it was not open last week could it be a virus flooding the network and stopping the other computers getting to the dhcp?
sorry if this message is in the wrong place.