Hi Tom,
All depends on your computer setup & operating system, and goals.
Probably you mean to have a recent backup Image so that if your system drive fails, you can just restore over everything back to a new HDD and you're back in business.
With Win-7, it includes a BackUp and Restore program, that takes an Image of your system, then incrementally backs files that change. Can run it yourself or put it on the task schedulerer. Default is weekly. Every 3 months it takes another full Image ad increments from there. Problems, click Restore, or if your computer is unbootable, use the Win-7 DVD containing the Recovery Environment to restore your stuff. Vey nice reliable program most folks don't even know about.
With Win-8 coming out OCt 26th, it includes the same Image BackUp applet, Plus a File History program that continuously backs up just data files which you can get anytime, Plus a new "Manage Storage" program that can Mirror your system Drive (Like RAID 1) continuously. If a drive goes south, it doesn't miss a beat and uses the "mirror" until you replace the bad one. Sounds like it covers all bases.
Then there's Commercial Cloning, and Imaging, and Backing Up programs, Like Acronis True Image ($49) which is very very popular and many people swear by.