Windows Home Server is technically available only as an OEM product
WHS: Where to get it

Microsoft do not sell WHS as a retail product, the software is available only to OEMs and System Builders, however it is widely available in PC stores online and offline.
The legitimate way to buy WHS is in a premade server
sold by an OEM. In Australia three main vendors are selling WHS boxes; Enspire,
Altech and Aus PC-Market. Buying a readymade WHS system gives you a warranty on
the system, and recourse should there be any problems.
However in the spirit of
APC, we’re going to roll our own (Microsoft do not offer any support for the
products should anything be amiss, you’re on your own). APC has found it relatively easy to obtain WHS on its own. A free 120-day trial of Windows Home Server can also be purchased from Microsoft.
Windows Home Server requires pretty basic specs to
get installed. The minimum system requirements are a 1GHz Processor (Pentium III
or better), 512MB of RAM and an 80GB hard disk drive. Of course an Ethernet
connection to link with the rest of your network is also required.
Bear in mind that if you’re using a system that includes graphics integrated
to the motherboard and only 512MB of RAM, then it will consume a portion of your
available memory and WHS will refuse to install. You can get around this by
running the installation with an extra stick of memory, then removing it after
the installation is complete.
Next: Choosing the Hardware