<rant> It would be nice if questions about Windows CE, (or any other subject,) were answered by people that actually know what they are talking about </rant>
Windows CE could run in the R-PI without any problem. (It supports ARM, MIPS, SH4 and x86 processors.) A 256Mbyte memory is huge compared with what many WinCE systems have.
To port WinCE to a new board/processor/architecture a new BSP (\"Board Support Package\") needs to be written or modified. A BSP contains mainly the drivers and bootloader, but the concept may be extended to include programming and configuration utilities etc., or anything required to get the system going.
The BSP is typically made not by Microsoft, but by (a) Chip manufacturers. (b) Board manufacturers. (c) Third party software houses. (d) Stubborn individuals. (Microsoft provides a couple of sample BSPs though)
Everything is documented in Microsoft’s web sites, but they make it sound easier than it is. Developing a BSP is *not* a small undertaking, in particular if not all the hardware details are known, as is the case with the Raspberry-Pi
Finally, it is wrong to consider WinCE as an another desktop OS alternative – It is not, it is an embedded OS. You may be using it all the time without knowing. Think GPS systems, elevator control systems, security alarms, irrigation systems, microwave ovens, printers, home automation, your TV and audio receiver, etc. Any modern piece of consumer electronic equipment may be running WinCE inside, just do not expect to see Microsoft or Windows logos on it. (I personally used it for industrial infrared cameras and remote controls for video distribution systems.)