It was a switch on the motherboard that was wrongly set causing the CPU problem.
I am going to install Windows again and should have the drivers on a CD to run the sound off the motherboard.
Then I will try to set up the network again!
Thanks Richard -
One question - the support guy on the phone said you should never plug or unplug your keyboard with the power on as it carries a charge. For my and other's benefit, is this correct? Apart from USB is it safe to plug/unplug any cables with power on? eg sound cables?
Thanks again