A Windows license has traditionally been one of the most expensive components when building your own PC, but that may change with Windows 8.
Windows 8 brings some major differences in licensing from past versions. Most notably, the so-called “Full Packaged Product” is out, and a new “Personal Use License (System Builder)” version is in. This lets users install the software on a computer they've built or as an additional operating system on a virtual machine or separate partition.
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Windows 8 System Builder Version May Come Cheap | PCWorld