Not sure if this is bad style, but I'm asking this question here because I couldn't find the answer elsewhere, and then I worked out one solution on my own. I'd be interested to see other people's solutions, but after a few days I'll post my own.
In my specific case, I'm running on Windows 7, but I'd be interested in answers for other/older versions of Windows. I realise that one answer is "install a version of Unix find, and then solve as for Unix", but I wanted a more "native" solution.
EDIT 2012-07-17: Clarification: by "automatically" I ideally mean something I can run as part of a script, rather than a GUI tool which does all the work at the press of a button, because I'll want to do this unattended.