I would go with the Java 7 solution over a native C extension. Using
directly from JRuby is usually a smoother path than working with C
extensions, plus if you use the Java solution your code should work on
any platform and wouldn’t be a Windows specific solution in case you
needed something different in the future.
I know that Headius mentioned JNotify some time ago as an option, and if
you don’t want to use Java 7 for it, this is probably another good
that you might be able to: http://jnotify.sourceforge.net/