Tom Luoma asked why I often recommend backing up your hard drive with imaging software, but never mention cloning software.
Cloning and imaging are similar in that they can both make exact copies of your hard drive's contents. When you clone a drive, you turn a second drive into a copy of the first. With imaging, you create a very large backup file from which you can recreate the drive's contents at a later date, either onto the original drive or another one.
(Most imaging programs also allow you to restore individual files from the backup.)
Cloning is obviously the simpler choice if you're upgrading your hard drive--provided you can get two drives working on the same PC.
Imaging makes a better choice for backup for the following reasons:
1. You have more media options. While cloning requires the dedicated use of another hard drive, you have far more options as to where you put an image. I've got several on one very large external hard drive. If you're willing to sit around and swap discs, you can even store an image onto multiple DVDs.
2. The better imaging programs, such as True Image, can do incremental backups, copying only the stuff that's changed since the last backup. This means it can do double-duty as your daily backup program.
3. Imaging is more secure. If malware infects your PC and it finds two Windows partitions, it may infect both. It's much less likely to infect a compressed backup file in a unique format.