In our domain we have a obsolete DHCP server showing in the list of Authorized servers that was decommissioned by previous network administrator and is no longer on the network. Highlighting this server and selecting UNAUTHORIZE causes following error. 'DHCP: mmc.exe Application Error. The instruction at "0x77f856af" referenced memory at "0x00000000". The memory could not be written.' Is there a way of unathorizing this obsolete DHCP server. The server does not appear in active directory. The IP address showing against this obsolete server in DHCP listing is also not a valid IP address for our subnet.