I have a client running Windows 2000 Server on their server. This server
also provides DHCP with the DNS information configured to point to the
server's IP address which is 10.1.1.1. At random times during the day, the
Windows XP client's which connect to this server will have their DNS IP
address changed to 192.168.15.1. There are no other DHCP servers running on
the network. I have never seen anything like this happen at other sites that
I have worked at. Does anyone have any ideas as to what may be the cause for
this?

There is, without a doubt, another dhcp server on their network. I would
suspect someone put in a dsl/cable router and didn't configure it to not
provide dhcp.

It is simple to ping the gateway of the 192.x.x.x subnet and then http to
the devices web interface.

I would agree that there must be another device providing DHCP, but I have
checked and have not been able to discover another DHCP server on the
network. Also, on these machines that have the wrong DNS, it is only the DNS
address that has changed. When I do an ipconfig /all, it shows the correct
IP address (on the 10.1.10 subnet) and the DHCP server that is shown is the
one on the 10.1.1.0 subnet.