When it comes to dual-booting between Windows 8 and Windows 7, the advice is the same as always: Install the older OS first, make room for the second OS, and then install the newer OS.
Dual-booting between two OSes—or more correctly, multi-booting between two or more OSes—is obviously a power user scenario, something that is not necessary for most. But this capability does let you install multiple versions of Windows on the same PC, albeit without the ability to run them side-by-side. (If you want to run two or more OSes simultaneously, you’ll need to look at a virtualization solution like Client Hyper-V.)
Setting up dual-boot is simple if you do it right, and I’ll assume here that you’re currently running Windows 7 and would like to installside-by-side on the same PC. Again, the rules haven’t changed: You should start with the oldest OS and then install the newer OS. So in this case, you’ll start with Windows 7 and then add Windows 8.
The “why” behind this logic is likewise simple: Each version of Windows comes with a new version of the Windows boot loader. And while newer versions of the Windows boot loader understand older versions, the reverse is not true. So while it’s technically possible to install Windows 7 on a PC after Windows 8, you’ll almost need to fuss with recovery tools in order to restore your ability to boot into the newer OS after the fact. Why bother?
Once Windows 7 is installed on the PC, there are two steps to follow to install Windows 8 in a dual-boot configuration:
1. Partition the disk. On a single disk PC, you will need to partition the disk first. (If you have two hard disks attached to the PC and wish to use the second drive for Windows 8, you can skip this step.)
In Windows 7, use the Disk Management tool (Start Search, disk management to shrink the current Windows Primary Partition to make space for Windows 8.
Then, create a new partition in the empty space, giving it a name like “Windows 8” so you can find it easily during Windows 8 Setup.
The wizard will prompt you to provide a drive letter (for Windows 7), and to choose a file system as well. Be sure to choose NTFS.
2. Install Windows 8 using the Custom install type. Now, you can run Windows 8 Setup. You can use optical disc- or USB-based Setup media, but you must do so by booting the PC from the media. (That is, do not run Setup from within Windows 7.)
Step through the Windows 8 Setup wizard normally. When you reach the screen that asks, “Which type of installation do you want?”, Choose “Custom: Install Windows only (advanced).”
Then, in the next screen, choose the new Windows 8 partition you previously created and continue with Setup.
Once Windows 8 is installed, you’ll find a new boot menu that appears when the PC starts up.
You can use this menu to pick between the OSes installed on the PC, or use the Change defaults or choose other options link to change the timer (which defaults at 30 seconds), choose which OS should be the default, or access the Windows 8 recovery tools.