Microsoft has just released the Technical Preview of Windows 10. It’s a very early build, so you wouldn’t be advised to run it as your main operating system, and while you could set it to dual boot, running it in a virtualized environment is probably a better idea.
The process is very simple -- all you need is a copy of Oracle VM VirtualBox and the Windows 10 Technical Preview ISO.
To set up Windows 10 in VirtualBox, launch the software and click New in the Manager window. In the Create Virtual Machine dialog box enter "Windows 10" as the name of the operating system. Select the Windows 8.1 as the version. (Choose Windows 8.1 (64bit) if you have a 64-bit ISO) Hit Next.
Set the Memory Size. The recommended figure should be fine. On the following screen choose to "Create a virtual hard drive now" and click Create. Accept VDI (VirtualBox Disk Image) as the hard drive file type. You’ll be asked if you want the new virtual hard drive to be dynamically altered (it will resize as required) or a fixed size. The latter option is faster, so select that.
Click Create and VirtualBox will begin building the drive.
When done, click the Start button in the Manager. In the "Select start-up disk" window, click the folder icon and navigate to the Windows 10 ISO.
Click Start and Windows 10 should begin to load. Select your language settings, then click the "Install now" button. Accept the license terms. Choose the "Custom: Install Windows only (advanced)" option.
Click Next, and the installation process will begin, rebooting as necessary.
When it’s done, you’ll be given the option of using express settings, or customizing things. It’s easiest to select the first one. Sign in with your Microsoft account (entering a code as required), and then finalize everything else.
It will start at the desktop, and one of the first things you’ll see is a Welcome to Tech Preview icon. Click this and an Internet Explorer window will open with tips and tricks, access to the technical preview forums and more.
If you want to resize the OS window, click View in VirtualBox and select Fullscreen or Scaled Mode.
The simplest way to shut down Windows 10 when you no longer need it is to click the X in top right corner of VirtualBox and select "Power off the machine".
Troubleshooting
If VirtualBox encounters errors and fails to install Windows 10, the first thing to do is make sure you’re running the very latest version of the virtualization software. I wasn’t and that was the source of my problem.
If that doesn’t fix the issue, go to System Properties on the host (not the virtual) PC, click the Advanced tab and click Settings under Performance. Select the Data Execution Prevention tab and change the setting from "Turn on DEP for essential Windows programs and services only" to "Turn on DEP for all programs and services except those I select".
OK that, restart your system and this time Windows 10 should install just fine in VirtualBox.