When we both connected an IP CONFLICT warning came up on both the PC's - what should I do about this please?
Do you have DHCP server enabled on the router - did you have to configure the ip addresses yourself?
My PC (using 10m cable) turned in 2.4Mb speeds on the speedtest site, yet immediately afterwards my son's PC (15m cable) turned in a speed of 3.4Mb. Is there anything I should tweak to bring the speed up on mine? I am Windows 7, he is XP.
Internet speeds can change, if you are using a webspeed test, it doesnt always stay the same.
you also said "My son's PC was wi-fi, but to ensure a better connection, I have now connected his to the router by ethernet cable as well."
make sure you have turned the wifi off, or disbabled it on the router i possible, and/or add wep/wpa security so nobody can hijack your WIFI!