There was a ReadyBoot subfolder under prefetch which it would not let me delete. I was able to delete that folder's contents, though.
On restart, I didn't notice any improvement. The HDD is still maxed-out at 100% for ages (which has been a consistent problem with Win10 on this machine since an update two years ago). I noticed that Search was high up the Disk usage list, which is odd, as Search has been disabled in Services!
(And is apparently still disabled. . .).
I've been a Windows user since 3.0, but I think the time has come to give up on this OS. I'm fed up of spending hours every evening fighting with it to try and get it to function well enough to do a bit of work. Especially as with each new version of Windows, MS makes it harder to access tools (even Safe Mode is challenging to get to without finding step-by-step instructions on the web first) and get "under the bonnet" to fix things. It's completely unproductive to use. The 2004 update is the most troublesome yet for me. I'll be rolling it back to the previous build as soon as I've backed-up my files, and will probably go further and do a factory reset. (I've read articles about the benefits of doing a full clean install without the manufacturer's bloatware, but I'll try the factory reset first - the PC was great when it was new - only becoming slow and unrelaibale a few updates along.)