Now that the Windows 8 Developer Preview has been released to the public, the easiest way to test this new OS is to install it in a virtual environment. Installing Windows 8 as your main OS or dual booting will allow you to fully take advantage of your hardware, but if you are looking to install Windows 8 without any major change to your system then VirtualBox is the way to go.

First of all, download Windows 8 Developer Preview from this link (32-bit or 64 bit depending on your system - the former if you're not sure):
http://msdn.microsoft.com/en-us/windows/apps/br229516

While your ISO download is in progress, you can get VirtualBox installed. Use the link for the latest "windows hosts" version and install it to your system:
http://www.virtualbox.org/wiki/Downloads

Once this is installed and the ISO downloaded, start VirtualBox and click "new":

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Then, click next to proceed:

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Now type in the Image name ("Windows 8" for example) and then choose "Windows " and "Windows 7" (x64 if you are using the 64-bit version) from the OS version dropdown. After this, click "next":

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You can now choose how much RAM to allocate to the virtual Windows 8 installation. You need to allocate 1024MB RAM (32-bit) or 2048 GB RAM (64-bit) at minimum, but can select more if you wish. Once you've chosen, click "next":

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The next step is to set up the virtual hard disk. 16GB (32-bit) or 20GB (64-bit) is the minimum amount, but higher is recommended if you are installing other applications. You can set this up to your own preferences, but we chose to create a 40GB dynamically expanding VHD using the following steps:

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Once you have followed through the above steps to create your virtual hard drive, you'll return to the main VirtualBox screen. Please select the Windows 8 VM and click "start":

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You'll now be presented with a "First Run Wizard" which leads you through the steps to booting from the ISO. First, click "next" to proceed:

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Now click the folder icon to browse for your downloaded Windows 8 ISO. Click "next" once you've done this.

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Finally your are presented with a confirmation screen with an overview of your first boot options. Click "start" to begin the boot process:

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Congratulations... in just a few moments the Windows 8 installation ISO will boot and you will be able to install Windows 8! You can read this installation guide to guide you through the Windows 8 installation process.

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Note:

I'd strongly suggest tweaking the standard configuration if your PC is relatively modern. Under settings, you can choose to enable 2D and 3D acceleration and check that hardware virtualization is enabled - this should speed up running Windows 8 in VirtualBox.