What is ZoomWindow, and why is it hanging?
ZoomWindow is a process that runs under "Accessibility" options (System Preferences). I had this turned on because of poor eyesight, and occasionally I would want to use a keyboard shortcut to zoom in on a specific area of the screen (think of the Magnifying Glass in Windows). I never stopped to think that it would be a constantly-running process that was using RAM and (very little) CPU, but when you're running a system as old as mine, you need every tiny resource you can free up. I turned off the "Accessibility" option in my Taskbar, but all that did was prevent the little "Accessibility" icon from showing up; ZoomWindow (or some derivative) would always be in Activity Monitor.

Just messing around with it, I discovered you can turn this process off by going into System Preferences, Accessibility, then selecting Zoom, and UN-checking the box that read "Use keyboard shortcuts to zoom"; also, you want to check "More Options" (from the Accessibility window), and make sure "Enable temporary zoom" is also UN-checked.

I had tried to Force-Quit the process countless times in Activity Monitor, and each time, it would close, then immediately come right back; by using the steps described above, I opened Activity Monitor and discovered that the ZoomWindow process was no longer running. I didn't even have to Force-Quit.

Hope this helps!