The computer is eleven years old, as is the laptop (although both have been upgraded with an SSD and extra Ram) - the company is no longer in business. I ran sfc /scannow and no problems were found, I also tried a refresh, keeping apps and files, but after over four hours the refresh failed and I was back where I started. I think it was a windows update problem as both the desktop had the same dated dll - the date of an update - and the same problem when I tried to use Foxit for PDFs, but sfc /scannow worked on the laptop. I have now set PDFs to open in Chrome and although the problem remains it is acceptable. Presumably the update had not been tested on PCs that old, although this is the first that has gone bad, and on both computers.