IP address conflict message on a bunch of computers

We have a single Windows 2003 Standard Server, which is our main file server and domain controller running active directory, dhcp, dns, etc...and we have a member server running windows 2003 and exchange 2003 standard edition.  We have about 40 client machines...mostly running XP pro and a few of them running windows 2K.

Yesterday, a handful of users (not sure how many exactly) received pop up messages on their computers about an IP address confict with another machine on the network.  I am wondering how I can possibly figure out why there was a conflict, and where or who this "offending" machine is?  In the past, I have occasionally seen two computer trying to share the same IP address, and they would get this error, but when I have about 10 machine all get it at the same time, it would seem that something else is happening here.

Also, I don't know if it's related, but I have a user accessing our network via our VPN software.  The problem is, the IP address he is getting from our DHCP server over the VPN, is NOT the address that our DHCP server or DNS server have listed for his client machine.  For example, if I ping his computer name, it responds and with an address of .79.  When I tried to remote control his .79 address, it connects to a PC here in the office across the hall from me.  When I looked at our Firewall to view his VPN connection statistics, it showed his internal ip address of .63.  
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jbobstAsked:
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DarthModCommented:
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DarthMod
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Jandakel2Commented:
I have seen this problem caused before because of some issue with WINS and NetBIOS....I had to disalbe NetBIOS over TCP/IP on the PDC Emulator of my domain and it seemed to solve the problem.  Take a look at your Event logs and see if you have any errors related to the aforementioned.  Usually when using dhcp for VPN Clients, you have a DHCP Redirector, which assigns from a separate range.  The old NetBIOS networks would broadcast to resolve IP's....try the nbtstat command and it's associated switches.....and let me know what turns up on both servers event logs.

JK
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jbobstAuthor Commented:
Sorry, but I just found the answer...we have a sister company in the suite next door, and they are a trusted domain.  Apparently, one of their machines had a real IP address conflict yesterday.  For some reason, some of our computers on our network had the error broadcast to them.

Thanks for the help.
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