That seems like the Nvidia drivers are causing issues. Where did you download them from and how did you install them?
Thanks for your reply. The downloads were given to me directly from Nvidia and I followed their instructions. They were quite helpfull in that they gave me several drivers to try but all downloaded OK and worked OK for the duration of the logon when they were downloaded. As soon as the PC is rebooted the problem starts ie I have to reinstall the Nvidia driver to get scetchup to work properly.