The system builder licence has traditionally been used by small OEMs (e.g. your local computer builder). In the past, home users were not allowed to use it, and had to buy the more expensive retail licence.
However, with Windows 8, Microsoft has changed their system builder licence to allow personal use:
a new license type called a Personal Use License for System Builder, which won’t be available until the General Availability of Windows 8 in October.
This last type replaces the outmoded and overpriced full package product and represents a significant and positive change in the way Microsoft licenses Windows.
With this change, people who build their computers themselves get the same, cheaper price. The licences are not quite the same; the system builder licence requires the builder to support the end user and has distribution clauses, etc.. The personal use licence is reminiscent of the old full retail licence.
The new personal use licence can be found here, the traditional OEM system builder licence can be found here.
At least here in Australia, you can probably buy a personal use licence from one of the smaller computer stores: http://www.pcworld.idg.com.au/article/440191/windows_8_launches_australia_-_can_buy_full_version/