Sorry, but AFAIK the kernel does not emulate Windows XP.
You might be thinking of
Wine which tries to translate system calls from Windows programs into equivalent Linux ones, thereby letting you run some 32bit Windows programs. However, this is nothing new and has been around for years (there are also two Wine spin-offs: Cedega which is more aimed at gaming, and Crossover Office, whichis aimed at making it easy to run and install office productivity applications).
Or possibly some of the hardware emulators you can run on top of Linux?