Thank you for the reply.
Just to make clear, these installations were done at very distinctly different times. And I'd forgotten one of the installations.
PC 1 - Windows 10 - was done in early December 2018
PC 2 - Windows 10 - was done in late December 2018
PC 3 - Windows 7 - was done in July 2019. This is the one that won't stay signed in for more than a few days.
PC 4 - Windows 10 - is actually PC 2. It got a new hard drive, clean installation of Windows, copy of prior files back on, but of course needs the software to be installed. Which went fine, but I never signed in.
So my questions:
1) Is PC 3 being Windows 7 the reason I'm always forced to sign in again to Office?
2) How might I force PC 3 to remain signed in to Office?
3) I have no idea what email address was used to sign into PC 4, when it was PC 2. Do I need a Microsoft or Live or Office address? Or will any of my personal email addresses work?