How to get DHCP setup?

HellO!
Currently, I have a range of IP addresses on a Cisco PIX for DHCP. I need to move the DHCP server to Windows 2003 Server. When I disable DHCP on the PIX, and implement the range on the Windows Server, machines that run out on their IP lease are coming up with limited or no connectivity. I have tried all IPCONFIG commands (flushdns, release, renew..)and rebooting the machine to get a new address to no avail. We have two PIX’s on our network and each held a range of addresses; I was successful moving one but not this one. Any help would be appreciated!