There are easy ways to make sure it doesn't happen to you. The simpliest way to safeguard your data is to store on two different media. If you happen to have 2 hard drives in your computer then use one for backup. Odds are slim that a computer failure would wipe out two drives. If you have a home network, you can be even safer by simply backup up files to a 2nd computer.
Zip drives are another relatively inexpensive solution.
For maximum protection, consider using one of the many free online storage website sprouting up. At Daily eDeals you can get information on about 20 websites offering as much as 500MB of free storage space. Parking your backup files on a remote computer means your data is safe even if your computer is destroyed or stolen. That can be quite useful or quite annoying. You can read the Excel help file for full details, but the short story is that the program will attempt to complete an entry based on other entries in the same column.