You can just restore the XP boot loader with fixMBR.
How to restore the Ubuntu/XP/Vista/7 bootloader (Updated for Ubuntu 9.10)

Or if you cannot find XP cd, you can install lilo's boot loader which works just like windows in looking for a boot partition and jumping to that partition to boot.

Restore basic windows boot loader - universe enabled
Simply open Synaptic and Settings > Repositories and tick the box against the Universe repo in the Ubuntu Software tab.
sudo apt-get install lilo
sudo lilo -M /dev/sda mbr
May show error messages about the rest of lilo missing, ignore, we just want MBR with bootloader.