How many HDD Drives do you have? Can you give a complete drive to Ubuntu or do you have one, and just hoping to share it in between the OS's.
If one, make a partition for 50% is the HDD size and install XP. After that when installing Ubuntu, you should see a portion that asks you if you want to install a GRUB boot-loader and when it comes to the partitioning section select "use remaindg free or unpartitioned space" and install ubuntu.
If two select the HDD and install it.
In any case, when I had this on my PC, I never had to worry, 'coz the PC always gave me that option from the start.
During the install of Ubuntu off of a live CD you will be given the option to partition your HD in order to setup a dual boot. Partition your HD during install and dedicate however much space you feel you will need to Ubuntu, by default Ubuntu may actually only allow a selected amount of space. But nonetheless, it's a pretty painless and easy process. The installation from live CD should be pretty self explanatory.