None of Windows CE, Windows Embedded XP, Nucleus, ecos, ThreadX, et al have any chance of coming any where near an 8051 - so this is entirely Off Topic!

The Windows products address entirely different applications from the others:

If yours is a media-rich application, needing extensive graphical user interfacing, cross-platform compatibility with PCs, etc, etc - then the Windows products may be appropriate.
There are embedded Linux equivalents;

If yours is a "traditional" embedded system, with little (direct) user interaction, little multimedia, and strict real-time constraints, you will go for a "traditional" RTOS.

If you're interested in the Microsoft products, why not visit
http://www.microsoft.com/windows/embedded
You can even get a free evaluation at
http://www.microsoft.com/windows/e...eeeval.asp

Personally, I never touch the stuff!