Quote: "My desktop shows up as JimsPC (as it should) but the user still shows up as MyPC, and my wife's laptop correctly shows up as JennysLaptop but still with user MyLaptop. Everything else works as it should."
This will be, quite simply, because the folder containing your user files (your "User" folder) is not renamed when you change your username.
Quote: "Surely, it's not beyond the wit of MS to have a simple change carried through consistently? I guess it must be."
Ha, it's most definitely not a simple change!

The REASON the folder is not renamed is because it would be a nightmare to manage. Heaps of programs would stop working, and shortcuts to stuff in that folder wouldn't work, and even things like playlists of your music would be invalidated
Quote: "and, in future, try to remember to set up a new user before doing anything else."
Good idea. I always make sure I select the right username (David) first time around after installing Windows, and if for whatever reason the PC has come back from a repair shop with a generic user already created, I'll delete that user and create my own one first thing.
Sorry I didn't see this thread sooner - I've been slacking off in checking for new GC threads recently due to having seasonal work (which has now finished, thankfully - one HTC One M8 phone, a Samsung 840 Evo 250GB SSD and $800+ later

). As I'm pretty much the most knowledgeable person on these forums when it comes to Windows (yes, I said it) it's pretty poor going of me to not check for Windows-related questions more often