This is an issue that I have seen often in Windows XP, but only ever in Windows XP. It's one I have never been able to solve. My Ubuntu connection works fine and I am currently using my Raspberry Pi to post on this forum.
Here is what I have tried so far:
After rebooting the router, before connecting my windows machine, I confirmed the list of devices and their ip addresses in the DHCP list by navigating to the router using my browser. When I connect the Windows machine it is assigned an ip that was not previously in use. It still gives me the ip address conflict message in the system tray.
I live where I work and internet here is off of a shared wireless access point. I am not in an optimal location for a connection, so I am running 1 Linksys WRT54G with DD-WRT in repeater bridge with an ip of xxx.xxx.x.252 to a second WRT54G with DD-WRT in repeater bridge with an ip of xxx.xxx.x.251. The outdoor access point is at xxx.xxx.x.253. The primary router is at xxx.xxx.x.254. After I rebooted the primary router, apart from the above mentioned devices there were only a single laptop on the system at xxx.xxx.x.1, my iPhone at xxx.xxx.x.4 and my RPi at xxx.xxx.x.125. My XP box takes xxx.xxx.x.2.
As mentioned at the beginning, Ubuntu works fine and RPi works fine. My iPhone also works fine.
Interestingly, switching to a different AP on the same network seems to solve the problem. Everything else is the same. The XP box still has the same IP, it's just connected with a different AP. That takes away the urgency of the issue, but if anyone has a solution to this conundrum I would appreciate learning of it. I have seen this error on other computers that did not have multiple APs strung together. It would be nice to be able to help them when switching to another AP is not an option.
The first thing to try would be a Winsock reset. From the command prompt, type netsh int ip reset c:\Reset.txt and hit Enter. Now type in netsh winsock reset and hit Enter. After that, close the command prompt window and restart your computer. It also couldn’t hurt to remove the network connection and re-add it.
It’s possible too that you have a loop in your network with the way you have your wireless hardware configured. Having two wireless switches configured improperly in one network can cause a loop situation to occur so your computer might be sending the IP conflict message to itself, thereby producing the same message, when in fact there is no IP conflict. The chances of a loop being the issue are even greater since you’re using the switches as bridges.
Another solution would be to verify your IP address, then shut down your computer. Go to another computer and ping your IP address. If you receive a reply, you have some hunting to do for a rogue device.
Keep in mind also the upcoming Windows XP End of Support date, which is April 8, 2014. If you meet the system requirements, it is strongly recommended that users make the move to a modern operating system such as Windows 7 or Windows 8 by then. A recent Springboard Blog post by Stephen Rose covers more about what end of support means and what you can do to prepare.