So you want to try out Windows 8. Excellent! But you're not foolhardy enough to try using a developer preview build as your main work/play operating system--you just want to dabble. We'll show you how to download and install the Windows 8 developer's preview on to a separate partition (or separate hard drive, if you have a spare). If you don't feel like mucking your hard drive with another partition, read How To Download and Install Windows 8 to a Virtual Machine for a less intrusive way to get Windows 8 up and running on your PC.
See also: Microsoft Windows 8 review
If you have a spare hard drive lying around, this process is pretty easy: Download the appropriate Windows 8 ISO from the Microsoft Developer Network site, burn it to a DVD, turn off your PC, slap the hard drive in your case, and connect it up to your motherboard. Then you just turn your system on and either press the key that allows you to access your motherboard's boot menu (or change the boot options in the BIOS so your optical drive is recognized before your main hard drive), and you should boot off your new Windows 8 disk. From there, just fire up the Windows 8 installer, and install the operating system to your new drive.
However, if you only have one hard drive in your system, you're going to have to get a little bit more creative in order to install Windows 8 without having to nuke your existing Windows installation. Welcome to the world of drive partitioning: In layman's terms, partitioning takes a hard drive's total storage and splits it into separate chunks of data. Your operating system then treats these separate data partitions as separate storage volumes.
Assuming you're currently running Windows Vista or Windows 7, open your Control Panel and open up the Administrative Tools screen. From there, double-click on the "Computer Management" option. When said window appears, look for the "Disk Management" submenu under "Storage" on the left-hand sidebar. Click that.
You'll see your hard drive (and optical drive) on the screen: Your primary drive should already be split into a "System Reserved" volume and your primary C:\ volume.
Right-click on the C:\ volume and select the "Shrink Volume" option, and then reduce the size of your volume by at least 16 gigabytes for a 32-bit installation of Windows 8 or 20 gigabytes for a 64-bit installation.
You'll now see a new, monochromatic, "Unallocated" hunk of storage appear next to your C:\ volume in the graphical display. Right-click on it, select the "New Simple Volume" option, and hit the "Next" button until you reach the screen for assigning letters and drive paths.
Feel free to assign your new volume whatever drive letter you most prefer. Click "Next" one more time and give your volume a witty name, and then hit "Next" (and then "Finish") to quick-format the volume as an NTFS partition.
Next, burn the Windows 8 image to a disc using a freeware app like Imgburn or CDBurnerXP, then reboot your PC, access your BIOS setup menu (typically by pressing Delete or another specified key while booting up), and change the boot order so your PC will boot from your optical drive instead of your primary hard drive when you restart your computer. Once your PC boots from the Windows 8 DVD, just install Windows 8 to your new partition (identifiable by the drive letter and name you picked out).
And here's an extra tip: Once you've loaded up Windows 8 for the first time, you'll be able to edit Windows' boot settings to make Windows 7 boot by default instead of 8 (which happens after a short time delay if you don't select an operating system yourself).