The Windows 8 Developer Preview (a pre-beta build) is available for download at
http://dev.windows.com/. Both 64-bit and 32-bit versions are available.
Warning: The Developer Preview is just that, a preview. Although relatively stable, it is not even at the Beta stage yet.
Note: Windows 8 Developer Preview System Requirements
Windows Developer Preview works great on the same hardware that powers Windows Vista and Windows 7:
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