Please see details and screen snaps attached to my first comment following. (I don't know how to add an attachment to this initial question.)
I newly installed Windows 7 on an HP Probook 4520s with 8 GB of RAM, for the purpose of running some legacy software. It downloaded about 1.2 GB of MS updates and they all installed successfully except the last one, KB4530734. - which is "2019-12 Security Monthly Quality Rollup for Windows 7 for x64-based systems"
I've rebooted and re-tried to install that update about 8 times. I've tried the procedure detailed at https://social.technet.microsoft.com/Forums/Lync/en-US/221a12a3-6cbb-4205-bcf3-150efa0ba437/repeatedly-failed-to-install-security-monthly-quality-rollup-for-windows-7-for-x64based-systems?forum=w7itprosecurity
Meanwhile, Microsoft Office 365 won't install either - it just freezes.
Thanks for your help!