If you're using Internet Explorer (like I do), by default a box will open when you start a download that will give you an option of saving the file or opening it. At the bottom of the box is a checked option "always ask before opening this kind of file." You probably accidentally unchecked it once, so IE no longer asks and does whatever it thinks is best (an annoying feature of all Microsoft products- software that likes to tell you what to do rather than doing what you want it to do). Unfortunately, I don't know how to change that. Microsoft products are also annoyingly opaque and resistant to a user changing settings.