Machine getting sluggish, slow or just a lot of pinwheeling?  It could be open applications using system resources.  In OS X (on Macintosh computers), closing an open window, often does not terminate the actual Application itself.  Take a look in the Dock (the row of applications on the screen).  Open applications have a small "light" under them.  You can terminate the application, by clicking the item with the light under it, and pressing Command Q.  The Command key is the key with what appears to be a freeway interchange on it.  The one that cannot be terminated, is "Finder" as this is the system resource that manages files and folders.