TCP/IP problem

Our Company is running NT Server4.0 with DNS,WINS & DHCP.Recently a client Workstation was removed from the network which had a static IP & it’s name was abc(say).
We installed a cisco router for RAS & assigned the same IP address that was used by the discontinued computer.
Every thing worked fine BUT, when I try to ping the router’s IP it resolves along with the old computer name.(I use ping -a x.y.z.81).
Where should I check this old entry?
Any ideas pleas!!!