First, congratulations to anticapitalista and the mutual aid crew for this extremely intelligent distro.
First problem: Live CD wouldn't boot on a Toshiba laptop. Tried it on a PC. No boot.
Solution: download the ISO again from a different source. The first one was probably a corrupt compilation or a corrupt CD burn.
Second problem: Installed AntiX on a new partition without installing a fresh Grub. Tried several configurations of existing Xubuntu /boot/grub/menu.lst to no avail.
Solution: reinstall AntiX AND new Grub, without the initrd option. Excellent: the new grub detected XP and Xubuntu.

Glad you figured it out! Welcome to antiX.