restart your computer and retry.
if that fails, reset the cc app: To reset the Creative Cloud desktop app, make sure it's in the foreground, then press:
Win: Ctrl + Alt + R
Mac: Cmd + Opt + R
if that fails, do a clean cc install:
uninstall every cc app including preferences, https://helpx.adobe.com/creative-cloud/help/uninstall-remove-app.html
then uninstall the cc desktop app, https://helpx.adobe.com/creative-cloud/help/uninstall-creative-cloud-desktop-app.html
clean your computer of cc files per http://www.adobe.com/support/contact/cscleanertool.html
restart your computer (don't skip this)
reinstall the cc desktop app, https://creative.adobe.com/products/creative-cloud.
if you're unable to install the cc desktop app at this stage, use an administrator account (solution 4 here, https://helpx.adobe.com/creative-cloud/kb/creative-cloud-missing-damaged.html)
use the desktop app to install your cc programs/trials
Thank you for your reply. I had restarted my computer several times and looked at what was running in Task Manager.
When I looked further down the list in "background processes" I found that "windows installer.exe" was there; admittedly only using minimal resources. I stopped the process and, hey presto, all OK. So it was Windows Update that caused my problem.