The most common oversight people make regarding the general maintenence of their computer is not creating backups of any irreplaceable files, such as photos, music, email addresses, spreadsheets, school work, and anything else of importance. It is assumed that the hard drive which stores these files will last forever, and if it does fail it can be repaired. Sadly, like everything else, hard drives wear out and will without a doubt fail, whether it's 10 years or 10 months. And no, they cannot be repaired, at least in the sense that you open it up, replace some part, and it's good as new. 99 times out of 100, when they're done, they're done. I always dread the phone call where I have to tell someone that their kid's baby pictures, wedding pictures, emails, everything...gone. And there's no getting it back. On rare occassions the data can be retrieved by a data recovery company, but this is a very long process that can cost thousands of dollars.
Your best bet is to back up these items BEFORE the drive fails, either on a CD, DVD, Flash drive, external (cloud) server or machine, slave hard drive or USB hard drive. It's simple and inexpensive, but can be time consuming to drag-n-drop everything from your directories to whatever medium your saving to, unless it's all in the default Documents and Settings folder (or "Users" folder in Vista/7), in which case just drop the whole folder onto the drive.
The "Files and Settings Transfer Wizard" in XP works quite well, as does "Windows Easy Transfer" in Vista/7.
The ideal backup, although more costly, is to clone your hard drive to a daisy-chained drive. IDE motherboards support up to 4 drives: 2 masters and 2 slaves, and SATA, SCSI, etc. boards support multiple drives. Buy a drive at least as big as the total space used on the existing drive, and use Norton Ghost or one of many other cloning applications available online. Some are free. For a little extra cash, buy an external USB hard drive to simplify things. Just plug it in and clone. If you're backed up to an internal drive, that drive can be an exact replacement for the failed drive, requiring no Windows reinstall or reinstall of any programs you've lost. It copies byte-for-byte and is literally plug n play. With a USB drive you'll need to clone the external drive back to the replacement internal drive. Not a big deal either. Weeks or even months of restoration are reduced to a half hour.