Try doing this:
**Please note, you have to reconfigure any static network settings after moving the below plist files
Open a terminal.
cd cd /Library/Preferences/SystemConfiguration/
sudo mv com.apple.airport.preferences.plist com.apple.airport.preferences.plist.orig
sudo mv com.apple.network.identification.plist com.apple.network.identification.plist.orig
sudo mv preferences.plist preferences.plist.orig
You have just preserved your old network system config files. Now go ahead and reboot the Mac (the Mac will recreate the needed plists) and see if the problem persists.
If you want to restore your Mac to its original state, just copy (cp) the .orig to the same directory with sudo and remove the .orig extension.
Tried that and it seemed to work, but after switching on and off the wifi network a couple of times to test it the problematic IP address reappeared on the system.
A systematic approach to turning everything off, restting the router and flushing the cache in each machine before reconnecting it to the network lead me to realise the connection to the XP PC was the causing the problem, but that is a topic for a different forum.
Thanks for all the advice all the same.