Turns out we'd only set up this backup system two weeks earlier, so we really lucked out. Let's make sure that you're lucky too. If you don't have a Time Machine backup, let's get you one. You'll need an external hard disk, such as this one:
Seagate GoFlex 1 terabyte external drive, $159.99 from Amazon, with free shipping.
You also need a Mac with OS X 10.5 or higher, and there are a lot of other reasons to have 10.5 or higher so if you're on 10.4.11 let's get you to 10.5 at least. You 10.4.11 people know who you are. (Unless you don't-- in that case, go to the Apple menu and choose "About This Mac" and see what it says in there).
Time Machine is Apple's own backup software, built into the system starting with 10.5, and it works like a charm. If you are on 10.5 or 10.6 you already have the software and it's already installed.
If you buy a drive that is formatted for a Mac (like the Seagate above), all you do is connect it and answer "Yes" when asked whether you want to use it for Time Machine. If you buy some other drive you will probably have to format it as Mac OS X Extended (use the Disk Utility, in the Utilities folder in the Applications folder). Either way, from then on you'll be backing up automatically every hour, nothing for you to worry about, and when your Mac's disk quits working the cost of the backup drive will seem like small potatoes. Recovering data from a dead hard disk will cost you at least $1,000 and usually more. It does not make sense to tempt fate here-- use Time Machine and have that backup for when you need it. Eventually, you'll need it. Make like a Boy Scout and be prepared.
