When you re-installed XP, your boot loader got wiped out, so now you can no longer boot linux. You did not state which distro you are using. You may be able to boot from your linux install CD and re-install a boot loader, either lilo or grub. I have used both. I found Lilo a little easier to get to work.
Post your distro, but also have a search on the board for similar problems. Search for Lilo or Grub to get an idea how to install them. Your linux install should be intact (assumimg nothing disastorus has happened to it) once you get a boot loader installed.