nslookup wrong data is shown

I apologise if i have posted the question in the wrong forum. i am currently an administrator and have about 150 windows systems running. the previous administrator turned out be be a real nightmare for me before he left. except the windows servers( static ips) the rest of the machines are assigned ip address through a dhcp server. when i look at the dhcp snapin in the active directory i can see system name and mac addresses assigned with ip addresses. if i do a nslookup for the same system it shows a different ip address and name. what i mean is, computer A on my dhcp list is assigned 10.10.10.100 . if i do a nslookup through command prompt for 10.10.10.100 it shows a completely different computer name Z and not A. and the funny thing is if i do a nslookup for computer name Z the ip address turns out to be completely different as well ( not 10.10.10.100)

running os : server 2003 standard sp2

its for most of the pc's in the network. has something been corrupted or have i been ippedd and macked