DHCP Address Leases gets 100% full

We use a Windows 2000 DHCP server and 2 of our subnets gets full in a matter of hours and the funny thing about it there was no host and the unique ID has a about 30 or more digits. We have to manually delete them so the IP addresses would be available again. Does anyone over here have the same problem? Is this a worm crawling in to our network? We tried fixing, rebuilding, restarting our DHCP servers but to no avail. Please help.Please help.