Finally figured it out. I'm posting here in case anyone else comes across this.
SOLUTION: Disable RunSwUsb service and set it to disabled. Alternatively, kill the RunSw.exe and SwUsb.exe processes (although they will come back). Or, see if they have updates to them. These are Realtek drivers (that I believe was installed with a WiFi dongle, but likely used by other Realtek devices as well).
HOW I FOUND IT: First, I shut down all running windows. Then, I went to the Processes tab in Task Manager. I tried stopping whatever processes I could willy-nilly in the "Background processes" section, checking the "Committed" value in Task Manager. When I got to "Switch USB2.0/USB3.0 for WinXP SP2+ ~ Win10 (32 bit)", showing a command line of C:\Windows\SwUSB.exe, ending the task caused the "Committed:" value in Task Manager to go down from 57GB to 17GB.
WHAT IT IS: RunSwUsb is a Realtek service, used at least for their wifi dongles. It sounds like RunSw.exe is the service itself, which starts SwUsb.exe which is the wifi driver. Some people have (guessed?) that the service is used to switch from USB3.0 to USB2.0 if overheating occurs, but who knows. I'm not 100% certain how the file got there: it was installed the day I received my WiFi dongle, but doesn't seem to appear in the driver package from the Realtek site. It is signed properly as Realtek. Hmmm.
THE WINDOWS PROBLEM: It is mind-boggling that Windows can allow a service to consume 40GB of committed memory while not showing it anywhere in Task Manager (or Process Explorer). Sure, some services will use 40GB (e.g. a RAM drive). But with the RAM drive, it at least shows up under "System". Right now, about 2 hours after a reboot, it has eaten up perhaps 250MB-500MB of committed memory (yes, "consume" is correct: it is RAM and/or paging file that other programs cannot use). Yet even with the advanced data in Process Explorer, nothing shows that (the highest is 80MB 'Virtual Size', but it was that high at startup, and doesn't go up).
Anyways, a thank you to Michael Taylor who helped me with some ideas. Even when you think you understand all the details about Windows memory management, there's always something else!