Resolved problem described in post one by changing from a wireless network to a wired one (powerlink, actually). Unfortunately, powerlink setup failed over the weekend, so I'm back to the wireless network with the "IP address conflict" message.
One thing I did notice was that the problem only occurred once I began using the range extender again, i.e. when I reset the wireless router and reconnected the access point, everything was fine, but when I began to use the range extender again because the router signal was weak, the "IP address conflict" message started again. This has caused me to conclude it's the range extender that is causing the problem. Is this feasible? I'm a computer dolt in general and wireless network dolt in particular, but one explanation that occurred to me is that my desktop computer is "reading" the router signal and the extender signal separately and that is what is triggering the "IP address conflict" message. I honestly don't believe the "IP address conflict" message is being caused by some other computer trying to access our wireless network because the problem happens whether my laptop is accessing the network or not and because our house is situated such that it is just not likely anyone is connecting to the network - because of parkland/new construction/vacant houses there is, by my estimation, a "computer-free" zone of several hundred feet surrounding my house.
Could it be the range extender causing the problem? If so, any solutions?
All assistance much appreciated.