Well it looks like Windows CE is supported on x86, MIPS, ARM, and SuperH processors, but it does not support the full Windows API. Windows XP embedded edition supports the full Windows API, but it only runs on x86, and the P-200 microprocessor (from what I can tell at least).
EDIT: It looks like the P-200 microprocessor uses the x86 architecture, so I was being a little redundant there.