First, you need to download the ISO image for the Windows 8 Developer Preview with Developer Tools here:
If you have an MSDN subscriber account, you can view other downloads (there is also
an x86 version with the Developer Tools)
Burn the image to a DVD using your favorite software such as the free ImgBurn . Or, you can burn it to a USB stick if you can boot off a USB.
NOTE: If you use a USB, be sure to format it as NTFS -- the install.wim file for Windows 8 is larger than 4Gb and will not copy to a FAT32 USB stick.
Next, you need to defragment your hard drive. This step just makes sense because
you're going to shrink your primary partition.
Next, you'll create a new Partition for Windows 8. For this, you'll need
to have at least 20Gb of free space on your main drive.
Open your Start Menu, and on the Computer option. Click "Manage" and then
open "Disk Management" in the left pane. This will show your existing
partitions.
Find your system hard disk in the graphical list that appears in the bottom pane.
Right-click on it and then click "Shrink Volume". Shrink it down so
you have at least 20GB of space left on the end of the drive, and click OK. Note
that the control to select the amount to shrink is denominated in MB, so to free
up 25Gb of unallocated space you'll need to enter 25000. Once this is complete.
you should see a new entry that says "Unallocated Space" to the right
of the shrunken main partition.
Then, click on the "Unallocated" block of that drive that appears and click
"New Simple Volume". Click Next on the next few windows until you get
to the "Format Partition" window. Here, give it a volume label like
"Windows 8" and click Next. It should format the drive for you. Now
you're all ready to install Windows 8.
Make sure your computer is set to boot from CD or USB, stick in your installation
media, and reboot. When you are prompted "Press any key to boot off DVD
or USB" do so.
You should boot right into the Windows 8 installer. It looks very similar to the
Windows 7 installer, so it should be reasonably familiar. Just pick your language,
hit "Install Now", and choose "Custom" when asked what type
of install you'd like to perform.
On the next screen, find the new partition you made on the list of drives. Make sure
it's the right one. Hit "Next" and let the installer do its thing.
When you're done, your computer should reboot into Windows 8. It may reboot
one more time after it does (do not "Press any key" if prompted to
boot off DVD or USB) , and you'll be greeted with the Windows 8 Start screen.
If you choose to enter a Windows Live ID here, make sure you have access to your
email and can confirm your computer—otherwise it might not let you boot into
it.
The final step is to make Windows 7 the default OS to boot into.
You'll notice when you first boot into Windows 8 that you see a new graphical
boot menu that lets you choose between your Windows 7 and Windows 8 installations.
Windows 8 will be the default, so if you don't manually choose Windows 7
from the menu, your computer will boot into Windows 8 after three seconds of
inactivity.
On the boot menu, click on the button at the bottom that says "Change Defaults
or Choose Other Options", and hit "Choose the Default Operating System".
From there, you can pick Windows 7 from the menu. From now on, your computer
will boot into Windows 7 by default, but if you want, you can choose Windows
8 instead.
For the sake of completeness, Scott Hanselman has an excellent post on how to create a Windows 8 VHD and boot off it here.
That's a little complex and scary to me, but I know Scott well and he is definitely
an expert on booting from a virtual hard drive.
That's it!