IPhone wifi router conflict issue!

Question

Something has gone awry somewhere, help!
My set up at home is thus.
One Ethernet router to which two PCs and one MacBook are hard connected, plus one wifi router connected to the Ethernet router, to which my iPhone connects to.
It has worked fine since I installed the wifi router.
This weekend, me and my other half have been away. I think it may also be the first time (but I don't recall exactly) that my iPhone has been turned off fully since I put the wifi router.
Anyway, we came back from our weekend away.
Other half put all the computers (well, his and mine) on, and the routers both come on too. His internets and mine were working. I had a bath then turned the iPhone wifi on and this, for some reason suddenly knocked his PC offline with the message:
"Windows-System error
There is an IP address conflict with another system on the network"
Now I'm not an expert but I assume this to mean two computers are attempting to sit on the same IP (I thought maybe the iPhone's wifi reset to router 192.168.1.1 - that's the IP it's telling me the connection is on.)
However his PC is almost definitely on .0.2 or .0.1 not 1.1 so I don't know why there's a conflict.
I just plugged the wifi router back in with the iPhone's connection off. His PC's internet works again. Very very odd. HELP!
My PC doesn't seem to be affected (not tried the Macbook yet, I expect it to be fine). He tried a different port on the router for his PC ethernet cable, and that made no difference! Baffling!
I'm not sure my iPhone is using the wifi when OH's PC is connected either, despite it saying that it is connected - I'm almost 100% definite that it's using the GPRS signal I get here, it's running very slowly.
I have tried changing the IP Static address to another number, that doesn't seem to have helped.

Answer 1

One common cause of address conflicts is when a system comes back online after an extended period in stand-by or hibernate mode, especially if the system was previously part of another network (e.g. an iphone brought home). A conflict will promptly occur if the address the system was previously configured with is the same as one that's already in use on the network. These conflicts tend to work themselves out. IP conflicts tend to eventually work themselves out in DHCP environments because systems issues repeated requests for valid addresses. But since the process can take a while and doesn't always work as planned, one way to hasten a fix is by manually entering IPCONFIG /RELEASE and IPCONFIG /RENEW from a command prompt (restarting a system accomplishes the same thing but takes a lot longer).
If the conflict persists even after renewing the IP address, it may be because your system's been re-issued the same address it already had and another system on your network is the source of the problem. You can get a clue about the offender by checking the System Log within the Windows Event Viewer (which you can get to via Start|Control Panel| Administrative Tools|Event Viewer) and looking for a DHCP error entry. The entry will list a network address (actually a MAC address) which you can use to identify the conflicting device, either by running IPCONFIG / ALL on a Windows system or checking the label on the back or underside of a non-PC device. Performing a IPCONFIG release and renew or restarting the other device should set things right. If not, restarting your router should do the trick as it will reset the DHCP server and force all systems to request new IP addresses.
1. Turn off everything on the network, ALL the machines and any other network device like network printers and NAS drives.
2. Turn off the router and modem.
3. Turn on the modem, wait for a steady data light.
4. Turn on the router, wait two minutes.
5. Turn on the network devices and computers, one at a time.
Enjoy!