newbie question ARM and windows

When you say you want to build one from scratch, what do you actually mean?

Do you mean:

1. you want to start with a chip like the lpc2000 series processor, etch your own 2 layer board(minimum here), design the peripherals around that and finally make an operating system from scatch for it, or
2. start with a minimal board with a processor on it(like the ones from olimex) and then build peripherals around it, and then uses say a OS like uClinux and recompile it for your needs, or
3. start with a single board computer heavy in peripherals, eg. CF interface , lcd interface, keypad interface, and much more. Maybe even have it pre- installed with some sort of OS, or lastly
4. build a computer, read as "put a computer together" usually a wintel box or similar.

Personally, I'm working from 2. Its not that hard, you just need to be decisive in what you want. Cos, you can't have everything on such small devices.

P.S. Sorry if my writing is not too lucid, have the flu at the moment and the medication is affecting my concentration.

Not for the typical meanings of the terms "run" and "Windows" you can't. You can run specialized versions of a system superficially similar to, but internally quite unlike regular Windows, on ARM CPUs in the 200+ MHz range. The resulting system is called a "Pocket PC" or "Windows Smartphone", depending on what version of Windows CE is on it, and whether it has a mobile phone built into it. The major problem in doing this yourself would be that individual developer licences for Windows CE essentially don't exist.

You may have to stretch the meaning of "possible" a bit, but: yes, it's possible.