Hi there,
I use a miniITX form factor single board computer based on a dual core Atom CPU and run Windows 7 Embedded as OS. It runs Mach3 a treat.
In order to run Windows (XP or otherwise) you will have to use an X86 platform (note: my assertion and maybe flawed!), ie Intel or AMD are the most
common.
Mach3 does not require an 'ultra processor', indeed it seems from the number of posts that people who try to use the 'latest and greatest' come unstuck and have
problems with the PP driver. Intels Atom range does fine. I believe AMD's G series 'system on a chip' will work too but I haven't tried one....yet.
If you require parallel ports then the choice is somewhat restricted if you determine that you want a built in port on the mother board. I have seen about fifty or so
from various makers with such a port. Commonly such boards will have a PCI expansion slot which allows for a card for a second port. This is exactly what I have
done, no probs.
I chose to use Windows 7 Embedded (Standard) as OS because I wanted to be 'master' of my own system, lol, Windows is not that simple! The local (New Zealand)
supplier who sold me the board also sold me the runtime licence for the OS. As it turns out the supplier was not in fact correct to do so, the licencing of Windows
Embedded is not straight forward. I have seen a number of suppliers who will sell you a built up system with the OS installed. According to their agreement with
Microsoft they are precluded from selling Windows Embedded standalone. They charge about $140 US for the licence component of their system.
If you wish to use a smoothstepper setup the just about every board out there can accommodate that, they all have at least one Ethernet port and up to four USB ports
on the board without expansion cards.
The smallest boards I have seen that include a PP port and a PCI slot are miniITX, 7"x7". If you don't require PP then look at the Minnowboard V2, about 4"x3".
My miniITX (UMG1 by Unigen in Taiwan) draws about 20W so I run fanless. In fact most Atoms or Gseries chips use less than 10W so fanless or a small fan is fine.
Most will run off a single 12V supply of a few amps. I use an ATX supply simply because I had a near new one lying around.