Have you backed up your data today?
If you lose access to your computer today, how much of that data would be irreplaceable? How much can you afford to lose? Do you want to lose 5 years worth of digital pictures of the family and kids, that could never be replaced? Do you have data or information that only exists on your computer?
Once the power's out in an emergency, or the house is on fire or during that lightening storm, it's too late to start backing up your data.
With the many affordable options for backing up data, the only excuse would be due to poor planning.
There are many options: USB drives, Secure Digital Cards, online backups, burnable DVD's, or external pocket hard drives.
Second tip: When you do backup your data, don't leave it on the desk by the computer. Put it in a safe place in another part of the house.