Thanks Rob. I have already tried to connect modem to router to PC as you suggest & no joy. Windows says it can't get an IP address - even when it offers to 'repair' the problem.
However, I CAN connect directly from router to PC (without the modem) and access the settings (under 192.168.0.1). As stated before, my PC has never had a problem connecting directly to the modem (which it is now - so the modem/PC must be working fine!)
I can connect directly from modem to 360 to Xbox Live. But as soon as I add the router to the mix I cannot connect to Xbox Live and I am told something along the lines that 'a lease could not be negotiated'.