Looking at that, seems you mis-remembered.
Grub is in the MBR, with its stage files in /dev/sda2
. So far so good.
When you re-install XP, it will indeed trash the MBR - easy enough to fix. Been a few months since I did an Arch fix-up, but I don't think it's the easiest for this. I use a Gentoo minimal CD, but any liveCD will do - Knoppix say. From a terminal, run the command "grub" as root, then
All done - just re-boot.