Installing an older Windows Version on a computer with preinstalled Windows 8 or Windows 10

New computers often have Windows 10 (or at least Windows 8) preinstalled. For some users this system is unsatisfying and they might prefer to install not only Linux but also an older Windows version. While older Windows can easily be overwritten by booting with a proper CD or USB-stic, this is prohibited in Windows 8/10.
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Installing an older Windows Version on a computer with Windows 8 or Windows 10

When booting the computer, change into the BIOS (usually done by pressing F2 while booting). Choose the "Boot" column and set the BIOS to "Legacy" instead of "Safe mode". Also check the boot order to ensure that booting from CD/DVD or USB stic is possible.