Anyone that knows me will know that I am paranoid about telling my clients to make sure they have a backup copy of all the important data on their computers. By backup copy I mean a copy on CD or DVD or a memory stick or another computer or an external hardrive or an online backup service or simply a printed copy.
How you backup depends on how many photos, emails, addresses, documents etc you have. But I can't stress too much the importance of making sure you have a copy of everything somewhere, just in case. If you are running a business or have photos or documents you really don't want to lose, then have at least 2 backup copies, with one being stored away from the computer.
I had yet another example yesterday of why backups are important. I was called to a computer that wouldn't start up, but was just going round in an endless loop of trying to start, turning off and turning back on again. It might be a problem with the RAM, it might be that the hard disc has failed. I sent the computer, and its distraught owner, back to the shop it came from. The physical problem can be fixed. But if the hardrive has failed and needs to be replaced the data will be lost - and the last backup copy was from January.
One day I've a feeling I will be able to write a book on 101 ways to lose your data and why you should back up. Meanwhile, If you're still not convinced I have written some of the real situations that happened to my clients on my main website. You can find them here. Cautionary tales for everyone. Don't let it happen to you - please.