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Microsoft has officially released the developer preview version of Windows 8 during the BUILD developer conference. The Windows Developer Preview is a pre-beta version of Windows 8 for developers, and thus is not suitable for those looking for stable or intending to run on production system, as the downloads include prerelease software that may change without notice.

3 versions of downloads are currently available officially from Microsoft on MSDN. If you decides to install Windows 8 Developer Preview, do read the Windows Developer Preview guide.

1. Windows Developer Preview English, 64-bit (x64)

Size: 3.6 GB
SHA-1 Hash: 79DBF235FD49F5C1C8F8C04E24BDE6E1D04DA1E9

Direct Download Link: WindowsDeveloperPreview-64bit-English.iso

Note: Includes a disk image file (.iso) to install the Windows Developer Preview and Metro style apps on a 64-bit PC.

2. Windows Developer Preview English, 32-bit (x86)

Size: 2.8 GB
SHA-1 Hash: 4E0698BBABE01ED27582C9FC16AD21C4422913CC

Direct Download Link: WindowsDeveloperPreview-32bit-English.iso

Note: Includes a disk image file (.iso) to install the Windows Developer Preview and Metro style apps on a 32-bit PC

3. Windows Developer Preview with developer tools English, 64-bit (x64)

Size: 4.8 GB
SHA-1 Hash: 6FE9352FB59F6D0789AF35D1001BD4E4E81E42AF

Direct Download Link: WindowsDeveloperPreview-64bit-English-Developer.iso

Note: All of the following come on a disk image file (.iso): 64-bit Windows Developer Preview, Windows SDK for Metro style apps, Microsoft Visual Studio 11 Express for Windows Developer Preview, Microsoft Expression Blend 5 Developer Preview and 28 Metro style apps including the BUILD Conference app

Windows 8 has the similar system requirements with 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

Source: My Digital Life