Thanks for the DISM tips, which, together with paring the page file down to 400 MB, have enabled me to finish up with just over 1 GB free, having installed Windows Live Mail and Jarte word processor. I'm now running this Windows 8.1 Pro system on the flash drive plugged in to my old HP Compaq nc6400, having managed to install the old ATI graphics driver with a bit of Windows-7-compatibility subterfuge.
It all works. I wasn't expecting to have so few problems, so I am at a bit of a loss as to what to do next. It's not often I can say that.