Simple question which I couldn't find an answer via google searches.
Is there any way to make the currently-focused/active notepad.exe window close with a keystroke e.g. CTRL+W?
Yes, The most common windows keyboard shortcut will work here: Alt+F4.
This keycombination may not work if there are unsaved changed in the document. In that case, you need to press Ctrl+S first to save your changes before you can close the window.
This keyboard shortcut key will close about every program unless the program itself added something to prevent it. Most games will also respect this key combination.
The difference between Ctrl+W and Alt+F4, is that Ctrl+W closes the current document. Given that Notepad does not have more than one documents open at a time, that key combination is not programmed in.