I tell people all the time to keep their digital images in more than one place...I never thought it would be my sister that I would have to worry about.  She's in the moving process and changing over computers and I'm trying to explain to her about having a backup system.  Not backing up to a cd/dvd...those are too easy to damage, loose and hard to upload large amounts of data, which pictures usually are.  Personally I have two external drives and to make it simple an external drive is like the storage part on your computer but it's sperate from the computer.  The first drive is one I use all the time and keep all my data on, I keep no files on my computers hard drive.  I then periodically copy them to my second drive as a backup to my backup.  When it's stormy out the drive gets unplug for fear of it getting fried and when I'm out of town it goes into the fire proof safe.  Here is the kind of external drive I use: http://www.samsclub.com/shopping/navigate.do?dest=5&item=392916 and it's 1tb (which is plenty of space) on sale at Sams Club for just over $100!  A side note this is also great since you can take it easier than your desktop and transfer files when you get a new computer later on.  Small price tag for peace of mind!