Following the general advice of using Time Machine, I loaded it on an external 500 gigger in June 2011. In October 2012 after having to restore my system I called on TM's help and found that the date line on the right side of the dark-sky page showed all the dates since June 2011 until November 2012 grayed out.
I thus lost all my bookmarks, addresses, music and iMovie and iPhoto apps although I had fortunately saved movies and photos to a second external drive. The worst loss is that of a manuscript to a book which disappeared from the desktop and its folder.
Thus I also lost the QuickTime MPEG-2 Component purchased in 2010. It disappeared last year and on July 19, 2011 I was instructed how to find and re-install it. Using those instructions this time, however, did not work.
I find that TimeMachine has been useless and I should have relied on making hard copies to DVD.
Last year I was even given a 90-day extension to my Applecare subscription after amassing a large number of case numbers, i.e. 233137931, 24041987, 240641989 and lengthy calls worthy of recording to a spread sheet.
I hope I can find some help here since an Apple store is some 400 miles away.