Unlike the Mac, Windows lets applications install files all over the system and program folders. The result of Windows allowing apps to install files or DLLS in many locations is it can wreck havoc with operating system stability. Apple has a better approach.
OS X confines application files to the application, system's library, and user's library folders. Some application preferences are system wide and others are specific to a certain user (which is why there are two library folders), but hints in OS X Lion suggest things are about to change.