Hello again,
I am delighted to say that my computer is now back up and running again!
When I read the recommended advice to uninstall anything I'd put on my computer just before this happened, I remembered that along with CCleaner and Sophos, I also recently installed a program called 'TeamViewer'. This allowed a friend to see my desktop from their computer halfway around the world, and to have control of the mouse, ability to type... full functionality as if they sat in front of my computer I think. This was to help me run CCleaner and also to have a look round at my settings and programs, as I had been having trouble with my computer previously.
So, out of the new programs - CCleaner, Sophos and TeamViewer - I thought that TeamViewer was probably the least robust, so decided to tackle this as my first step. In safe mode, I opened TeamViewer and unchecked the box to load the program at startup. I then shutdown and started up again, and everything loaded as normal
To check that this wasn't a random occurance, I decided to check the box again to load TeamViewer at startup, and see if it went back to the old problem. When I did this in the TeamViewer options and clicked 'ok', a window came up saying that 'Windows cannot access the specified device, path or file. You may not have appropriate permissions to access the item'. This was accompanied with a pop-up message from Sophos saying 'File "C:\Program Files\TeamViewer\Version4'TeamViewer_Service.exe" belongs to adware/PUA
TeamViewer (of type Remote administration tool).'
From this I gathered that Sophos had a problem with TeamViewer, as I guess that sometimes it can be used maliciously. So I didn't get to finish my test of whether it was a random occurance, but I did learn that perhaps there is a conflict between TeamViewer and Sophos. I am happy to leave this as a mystery because I am happy to leave TeamViewer set NOT to start at startup, or perhaps to uninstall it altogether, I haven't decided yet.
Thanks again for your help, Andavari. Do let me know if you think it's important that I get rid of the TeamViewer program altogether, and/or if I should still use a recent Restore Point (I assume this is the same as doing a System Restore?).