The first thing I would suggest is checking if there is an unauthorised DHCP server running on the network. It can be quite surprising what equipment can act as a DHCP server. A simple way to check is to disable the DHCP server service on your Windows 2003 server and renew the IP address on a PC. If the IP renews successfully, then there is a DHCP server running on another device that needs to be disabled.
If this happens, here's a couple of things to check:
- Make sure DHCP is disabled on your router.
- If you have any Wi-Fi access points, check their configuration, as some Wi-Fi access points have a built-in DHCP server on by default.
- Windows XP has its own DHCP server built-in, which can be activated if a PC is used to let's say share a USB modem (cable, satellite, etc.) over the network. So make sure "Internet Connection Sharing" is disabled on PCs and especially laptops.
If the IP fails to renew after the DHCP service is disabled, enable it again and try the following on an affected PC: Open a command prompt and type in "ipconfig /release" to force it to release its previous info it got from the DHCP server, then type in "ipconfig /renew".