Let's start with the "Developer Preview"

Welcome to the world of the latest Microsoft operating system: Windows 8, a revolutionary and refreshing iteration in the changing world of personal and enterprise computing. You can start with downloading the Developer Preview directly from the Microsoft website. Do you have a good connection, some time and space for an ISO image which is 4.8 GB? So you are ready. The next step is to copy the ISO image into an USB flash drive and make it bootable and that you can do with the
Windows 7 USB/DVD Download tool. All the steps are clearly explained by Windows guru extraordinaire Scott Hanselman in his blog: Guide to Installing and Booting Windows 8 Developer Preview off a VHD (Virtual Hard Disk). It was very helpful for me and I am running Windows 7 and this Developer Preview in my laptop. The latter within a 15 GB .vhd file, but make sure you have plenty of space or you won't be able to start Windows 8. The cool thing is that it will run as it was installed in a separate hard disk partitition or in another hard disk. When you start your computer you will see a new boot entry in the boot configuration data (BCD) store on the system partition (the BCD is there since Windows Vista instead of the old boot.ini). Give it a try and start appreciating Windows 8. It's worthwhile. It's another reincarnation of Windows 7, sort of.