|Do you have more than one DHCP server running?|
If you have more than one, shut one down and see if that doesn't fix the issue.
If you only have one DHCP server running, then I'll need some info.
Are you using a SOHO Router?
What device is the DHCP server running on (ie: router, server)?
What are the DHCP server settings (ie: Scope, Subnet Mask, DNS, Default Gateway)?
Finally, on a client open a command prompt window (ie: Start >> Run >> type cmd and hit Enter) and perform the following command:
Once you've finished, copy/paste the output into a response on here so we can have a look.
For info on how to copy/paste from a command prompt window click on my name above and read my "how-to" guide titled, "Post the output of ipconfig (version 2)"
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.