The normal way to install a dual boot is to put XP on first then Linux, so you need to repair the GRUB bootloader.
Reinstall GRUB by doing the following:
1. Boot off a rescue floppy or CD
2. Run grub and enter the following commands
3. root (hd0,0)
4. setup (hd0,0)
5. quit
6. reboot
Or, you can
1. Boot off a rescue floppy or CD
2. run: grub-install /dev/hd0
3. reboot
This will reinstall GRUB onto the first partition of the first hard drive.
Change the hard drive number as appropriate for your drive configuration,
but the above is correct for most configurations, this assumes you have an IDE drive (PATA).