Where is your important data? On your hard drive? Good. Where else? If your answer is "What do you mean, where else?" you're violating the First Law of Safe Computing: Never have only one copy of anything. If your documents, address books, worksheets, and photos are worth keeping, they're worth backing up.
Thanks to the proliferation of writable CD and DVD drives, USB storage devices, and hard drives the size of Wisconsin, backing up has never been easier. Whether you back up to an external hard drive (easy, dependable, but not cheap); an extra partition on your existing hard drive (easy, cheap, but no protection against a hard-drive crash or a stolen computer); or CD-RW, DVD-RW, or DVD+RW discs (dependable, cheap if you already own the drive, but not quite as easy), you should be backing up.
One problem with backing up to CD-RW (and to a lesser extent, DVD), is that one disc may not hold everything you need to back up, forcing you to sit at your computer waiting for a cue to swap discs. Another problem is that Windows doesn't see a CD or a DVD as something it can write to (Windows XP does--sort of--but not in a reliable way). Most of the backup programs I recommend here can't write directly to these discs without the use of special packet-writing software that runs in the background, disguising your drive as a giant floppy. The good news is that such software probably came with your CD-R/RW or rewritable DVD drive. If you can write to CD or DVD by simply dragging a file to the appropriate drive icon without any additional steps, you're running the correct software.
If you can't, you may need to search your Start menu for the packet-writing program that came with your drive (or with your PC, if the drive came with that). If your CD/DVD-authoring software is Easy CD Creator, the packet writer is called either Drag-To-Disc (for version 6.0) or DirectCD (earlier versions). If you have Nero software, it's called InCD. All you need to do once you've found the program is launch it, and your discs will be as easy to write to as a floppy disk. You can either launch the program before you back up every day, or drag the packet-writing program's icon to your Start menu so that it launches every time you boot.