I downloaded and installed a new Nvidia graphics driver after which when I booted up, the computer just went to a blank screen. I tried booting into safe mode by holding down Shift+F8 but had no success. In the end I forced a shutdown by holding the power button down when the windows logo appeared, and the little thingy was going round. I then restarted and fortunately it went into repair mode and suggested a system restore. Happily this solved the problem! At one stage I thought I was going to have to boot with the win 8 DVD, I am glad though I didn't have to and it all worked out. What a convoluted way of doing it though! Might there be an easier way? If anyone has any suggestions I would be interested.