In hindsight this seems obvious, but several Google searches did not reveal the answer so I am documenting it now in the hopes that others who are stymied by this can do something about it, too. As always if there’s a better way to do this leave me a comment. Thanks!
I was being annoyed by Windows importing and tagging the photos on my iPhone, often several times per sync. I want to be able to plug my iPhone in, have it sync, and then leave. Instead I’d get “No new pictures or videos were found on this device” dialogs, requiring that I click OK. If I didn’t click OK fast enough I’d get “This device is already being used by ‘Windows'” as it tried syncing a second time:
Turns out the fix is easy. With your phone connected to the PC go into Control Panel, then into AutoPlay. Scroll down to the bottom, under Devices, and change the action from “Import pictures using Windows” to “Take no action” (or whatever you want it to do). Then click Save:
That’s it. Like I said, easy to do once you know where to look.