does xandros use Grub or Lilo ?
anyways, boot a linux rescue disk, then add Xandros to the boot menu manually.
if your suing grub, you needd to edit the file /boot/grub/menu.lst
add the tiles..
title Xandros Linux
kernel /path/to/kernel root=/dev/hda1
replace (hd0,0) with the partiton that the kernel (boot foler) is on.
replace /path/to/kernel with the kernel file relative to the partiton you entered for root.
replace initrd with the path to the initrd file (same path as the kernel) if you have one... this is optional.
replace root=/dev/hda1 with the block device for the root partiton.
the kernel will be called bzImage or vmlinuz or linux depending on the distro.
there are loads of guides on this site about grub and lilo.