In case anyone has this problem here's the advice I got from a helpline. Switch the PC on, as soon as the PB logo appears hold the power button to switch off. Repeat a few times till and this will trip the message 'preparing auto repair'. It should load a message describing the error, select the 'restore' option'. Leave the PC switched on while it restores, could take a few minutes, should then load the login screen. Sounds like this problem is caused by an inability to process a windows update.