Mixture
Whilst I agree that a "system builder" may indeed require MS reseller registration or whatever, I do not see where that applies to an "assembler".
It is all fairly academic nowadays since I tend to use Linuxmint when I build a computer - it is faster, has a smaller footprint and doesn't seem to suffer from slow down with age.
A new build with mint can be up and running within a short time as there is no need to install all those drivers from the install cd, that comes with the motherboard.
Mint, and other linux systems, tend to have the necessary software, office suite web browser etc, already on the installation media, a usb stick.
One can also test a build as most Linux distributions will run from a usb stick even without there being a hard drive in the computer.
And then there is the virus issue!!!
The only time I now build an MS system is if the end user needs a gaming system - I tend to use an OEM copy of Win7 for those.