Assuming it is not a WUBI installation, but a normal one on its own partition, +1 to what rhlegion suggested. Just delete or format the xp partition using gparted. You can then either expand an existing partition to cover that space, or use it as a separate data partition for Ubuntu. If you choose to format, rather than merging it in an adjacent partition, I'd suggest to format it as ext4, since the system is going to be Linux-only.
The gparted tool is already included in the Ubuntu live cd.
If you choose to 'expand' a partition to merge the existing XP partition in it, BE CAREFUL to NOT move the LEFT edge of your 'root' partition, as doing so will render the installation unbootable (although that can be easily fixed by reinstalling just GRUB, but why bother?). Also, be informed that resizing a partition from its LEFT edge often takes enormous amount of time, even if the partition is empty, while deleting-recreating will be done almost instantly.
To avoid accidental mistakes, it is better to install and run gparted from within the existing installation. While connected to internet, you can install it with
sudo apt-get install gparted