If XP starts OK on this PC then I would suggest you have not installed GRUB when you installed Ubuntu, or it has been corrupted since installation. Do a search for Supergrub and download disk 1, burn it to a cd and boot from it. A little trial and error is needed, make notes, and you will get a bootable Ubuntu system.
In Ubuntu in package manager search for Grub and install a package that gives an option to edit boot menu.
When you reboot the PC will boot to Grub and this will give you the choice of XP or Ubuntu. Ubuntu will be the default but this can be changed. Any issues can be resolved with the use of the supergrub cd so you should always be able to get back to a working XP system at least.