Windows finally releases the Windows 8 developer version that people can try by free. Except using DVD you can also install Windows 8 straight from a USB Flash drive. You need Windows 7 USB/DVD Download Tool to begin create Windows bootable on your USB flash drive from the Microsoft Store. Although it's for Windows 7 but the tool also can be used for creating a Windows 8 bootable USB because both have same DVD bootable structure.
How to Create Windows 8 Bootable USB Flash Drive Installer:
You need to download Microsoft's Windows 7 USB/DVD Download Tool then install it and Windows 8 ISO Developer Preview.
- Insert a USB flash drive that has more then 4GB storage capacity in your computer USB port.
- Launch Windows 7 USB/DVD Download Tool.
- Browse the Windows 8 ISO on your computer

- Click "USB device" button when prompted to choose media type.

- Then choose your inserted USB flash drive location and click Begin copying

- The tool will creating bootable Windows 8 USB installer flash drive using the Windows 8 iso.

- If successful you will see the message "Bootable USB Device created successfully"
- Now you can can restart your Computer, boot from the USB drive and follow on-screen instructions to install Windows 8 Developer Preview.
System Requirements for Installing Windows 8:
- 1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
- 1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
- 16 GB available hard disk space (32-bit) or 20 GB (64-bit)
- DirectX 9 graphics device with WDDM 1.0 or higher driver
- Taking advantage of touch input requires a screen that supports multi-touch
Credit: Image credit to techmixer.com