While I have not specifically compiled a new 2.4.21 kernel on my Xandros
Desktop 1.0 system, here are some of my thoughts:

   In the /patches/ directory of this site, are each of the patches
   (numbered in the correct order), that Xandros uses to create their
   2.4.19-x1 kernel.

   In the /newpatches/ directory of this site, are the new NetTraverse
   Win4Lin patches for 2.4.20 and 2.4.21 kernels.  They replace patch
   009 and 010 from the /patches/ directory.

   Some patches, such as the 007pegasus and 003sis900 may simply not be
   necessary for a new kernel.

   001 splashFX is Xandros' Bootup splash graphics pages.  While not
   technically required, your bootup process will not be as clean without
   it.


If you do install a new kernel, the following (possibly bad) things will
result.

   Any device drivers or kernel updates from Xandros will almost
   definately fail to work or even install.
   
   Currently installed drivers that were seperate packages from the
   kernel will likely fail to operate.  To verify this exact list,
   see the pcimap tables in /etc/devices/ext/ or the output of the
   following command: dpkg -l "*2.4.19*"

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Just some personal notes here :-)

fakeroot make-kpkg clean
fakeroot make-kpkg --append-to-version=-j1-win4lin --version=1 kernel_image