If you want to dual-boot Windows 7 and Windows 8, the Upgrade Assistant is a good place to start
If you want to run Windows 8, but you'd rather hang on to your existing Windows 7 installation, it's relatively easy to install both operating systems on your PC with the option to select between them at boot time.
Note that you'll need to purchase a full version of Windows 8, rather than the cheaper upgrade option, to do this.
A good place to start is with the Windows 8 Upgrade Assistant, which will check your PC for compatibility before you commit to buying the new OS. It also gives you the option to purchase Windows 8 directly as a download, and the ability to create a bootable USB flash drive or DVD from which you can install it.
Having bought and downloaded the full version of Windows 8, and created your bootable installation medium, you can prepare your hard drive to accommodate the additional operating system. To do this you will need at least 20GB of free storage space; in fact, we recommend you have substantially more available – perhaps 10 times this amount.
You will now need to shrink your system volume to make space for a Windows 8 partition. Go to the Control Panel, then open Administrative Tools, Computer Management, Disk Management. Right-click the drive on which you wish to install Windows 8, then select Shrink volume. You should now enter an amount by which you wish to shrink the volume – at least 20GB.
Having set up your hard drive partition, you can now boot your installation media to begin the installation process. Specify that you want to make a custom install, then select your newly created partition as the installation location for Windows 8.
Once the installation is complete, Windows will allow you to choose between Windows 7 and Windows 8 at boot time.