I had just purchased a couple of neat small external hard drives through eBay here in Australia. I overdid it but got the eBay bug. Just before I went on a two week holiday I was mucking around on the computer and backed up most of my important files on one of the drives and took it with me. On returning I had a computer crash and had to reinstall Win XP from scratch.
While I didn't need very much of the back up I made, it was a lesson in how vital a backup can be. Now all I've got to do is make a backup plan and stick to it.