One of the more confusing ideas for non-specialists is that of using and/or installing a Linux-based system on a PC that already runs MS Windows. Most PCs (sadly) are sold with a version of Windows already installed on the hard disk. In almost all cases, this pre-installed version occupies the entire hard disk, even though most users don’t need anything like the whole capacity of today’s large disks. To use Linux on such a system, you can of course leave the hard disk untouched and load a Linux system from a removable device such as a CD-ROM or USB stick; such Linux-based systems can easily access the files on the MS Windows hard disk. You can, however, also add a Linux-based system to your hard disk and define at boot time whether you want to use this or the MS Windows system. The process of fitting 2 or more different operating systems on one disk drive is quite straightforward, though it has to be done correctly so you don’t end up erasing the Windows system entirely, which may or may not be what you want. Let’s start by looking at the boot process, look at the different formats operating systems use to store the programs and data, and then look at how to set up this dual system.