You back up your computer, right? Right? Come on, be honest.
Truth be told, few people backup their computers at all. Whether you fall prey to malware, your hard drive crashes, or if your notebook gets lost/stolen, all of these situations can be nightmares. Losing all of your data because it isn't backed up can be a disaster.
Financial records. Music. Photos and videos. Genealogy projects. Work documents. Some of these things can be backed up with a site like the sublime Dropbox.com, but such services are a poor substitute for a full backup of your personal computer.
If you've been meaning to backup your PC but weren't sure how to get started, read this excellent Lifehacker.com post (complete with video), and you will no longer have an excuse.