If you got a message that Ubuntu was successfully installed and asking you to reboot, then your problem is most likely that mentioned above. You did not install the Ubuntu Grub2 bootloader to the master boot record and if you are still booting xp, the xp boot code is in the mbr. You can manually configure the xp install to boot Ubuntu or you can install the Ubuntu Grub2 on the master boot record. I think you can guess which would be easier?
See the link below, Section 12 in particular for several methods. I would suggest reading through this before doing anything: