Yesterday my Windows 8 messed up after I installed AMD CodeAnalyst. I was left with a computer that took ~5 minutes to boot as compared to 18 seconds. First thing that came to mind was Refresh the system, which worked great but I lost all my installed applications as I never had any custom refresh image. Also one of the applications that I wanted is not currently supported to be installed on Windows 8 but works if I upgrade, with refresh that was gone. And I ended up redoing the installation. You can imagine I had fun time installing Windows 7 and then my applications and then upgrading to Windows 8. This was all night job.
All of this would have avoided if I had created my custom Refresh Image, as I learnt from Mr. Sinofsky Windows 8 has built-in options to Refresh and Reset your computer without much geekiness.
These options are quite useful when your PC is all messed up and you want to go back to sane state. Refresh will just redo the installation while keeping all of you data files but will remove all the applications and apps, while Reset is brand new installation.
Now most of us would rather Refresh rather than Reset our PC, but the few would like a pristine environment and would Reset.
I won’t duplicate what others have already said and done, but would rather point to links that can provide more detailed information on this: