By re-installing xp, windows overwrote the MBR, where the first part of grub was. Be assured, Linux is still there. If you have a boot disk, or install cd to boot from, you can still access it.
This question comes up almost every day. Do a search of these boards on the keywords 'grub mbr' and you'll probably get a ton of answers on how to fix it. (but first, you'll have to be able to boot Linux - as mentioned above).