I earlier posted this question in the Windows 10 Forum, since it deals with Windows Update. However, since the root of the issue seems to be MS Office, I thought I would repost it here to see if I can get a more definitive answer. The original post is as follows:

"I think I sort of know what is causing an update to fail repeatedly, but don't know what to do about it. I have Windows update set to update other Microsoft products as well. At one time, on my previous computer, I had Microsoft Office 2007 installed. I subsequently upgraded to Microsoft Office 2010. I used a program called PC Mover to move Office 2010 from the old computer to the new one. That worked fine, and Office 2010 seems to be functioning normally. However, Windows Update keeps trying to install Security Update for Microsoft Office PowerPoint 2007 (KB3213642) - Error 0x80070643. I suspect that this is caused by some remnant of Office 2007 that was left on the old system after upgrade to Office 2010, and moved to the new system with Office 2010. However, I have no idea how to identify, or find the suspected remnant. In checking the Installed Applications list, I DID find Microsoft Office 2007 Interop Assemblies installed. Could this be causing the problem? And if so, is there any reason for me to keep Microsoft Office 2007 Interop Assemblies when I no longer have Microsoft Office 2007 installed?"

So really looking for an answer to the Interop Assemblies question above, but also to what else associated with PowerPoint 2007 might still be causing Windows Update to try to install an update for it.