Question Author
Thank you very much, VHG.
I really have done almost everything you suggest already. I run disk cleaner and defragment regularly, and have recently run Ccleaner (virtually nothing to remove). I clear the temporary internet files and empty the recycle bin afterwards as a matter of course, and I have no video files or itunes. There are only a few things in My Documents, and together they don't amount to more than a few hundred KB.
I have an F drive on which I store all my digital photos, and a small amount on G which I use for family pictures. I don't know about D - I'll investigate that.
I think it may be time to buy a new computer....