Keef, both PCs are connected to the
same aerial socket via a 3-way adapter (only 2 outputs used). Although I have tried switching my daughter's and my own cables (with the effect that mine still works and hers still doesn't) I haven't yet tried connecting her cable directly into the socket - will do that when I get home this evening. That said, it would only affect the TV reception side of things, not the concurrent internet and networking issues.
Speedtest is a good idea - I'll try that too. I have also read elsewhere that the software or router firewall blocking access to https: addresses may be responsible, or even that the system clock may be wrong (time and/or date). Apparently, all these are known possible causes of the 'Unknown Discovery System Error 14' message.
Still can't understand why Media Center can't find the internet or log onto the network when its host PC can, though.
Gadget 