When reading either of these questions or the EmacsWiki article about mapping Caps Lock to Control in emacs in Windows, the best answers seem to involve the registry. My que
I know this does not really answer your question, but Trey Jackson basically explained the problem. To detect control key alone requires low-level keyboard hook on Windows, which I don't think Emacs implements.
One way (involving external program, but does not require admin right) is to use AutoHotkey. All you need is the following 2 line script:
CapsLock::Ctrl
LCtrl::Capslock
You will need to run this script whenever emacs is open, but thankfully Autohotkey is pretty low profile application (usually takes about a few hundreds Kb in memory only). You can for example execute this script in your .emacs so that your control key and capslock key are swapped whenever emacs is opened.