OK, we can try using "testdisk" on your Windows XP partition to fix the boot sector, so first boot into Ubuntu, enable all your repositories in System > Prefs > Software Sources, and then download and run testdisk:
After starting testdisk, choose "no log", choose the correct HDD and "proceed", choose "intel", choose "advanced", select the Windows XP partition, choose "boot", then choose "Rebuild BS". After you are done doing the "rebuild BS" in testdisk, reboot and let me know what happens.
sudo apt-get install testdisk
Also, please post the following: