Attemp to Understand Partition Cloning & Restoration

Since friends started to ask for my help to restore their Windows since college, I have wondered how to restore a PC without formatting the partition but just recopying a partition image done earlier which i never had done. With that, there is no hassle to reinstall the drivers and common software every time the PC needs to be reformatted.

To my understanding, it is possible if the PC is still running and you can log in to the Windows OS. But what if you can't enter the windows? There must be a boot-able CD / Flash Drive / Files that could initiate the recopying of the image files without the knowledge of DOS.

Hope to continue with the solution here someday ;)