transfan :
Well what could you get that hasn't been prebuilt?
there are sites like scan.co.uk
and ones in america as well
that let you select parts and they will build it for you and install os
this is a much better way to do it if your not sure about building your own.
building your own pc is usually a lot cheaper and not that hard to do
most motherboards come with a book telling you how to install everything from power cables to cpu.
the hardest component to choice is a graphics card and psu
graphics card because the numbers mean nothing unless you know what to look for. e.g. a hd 6990 is better than a hd 7670
psu because choose a bad one and can kill parts eventually