The first thing you need to do is to find out how much space you need to allocate to the Windows XP partition in order to have enough space for Windows XP, all your stored files on Windows XP and at least 15% extra space.
In the Ubuntu installer you will see an Allocate drive space window. Select the first option: Install Ubuntu alongside Windows as shown in the picture below.
Next in the Ubuntu installer you will see an Install Ubuntu alongside Windows window with a slider that you can slide back and forth to allocate drive space between Windows and Ubuntu. You should allocate enough drive space for Ubuntu to include the Ubuntu operating system (at least 5GB, but 10GB would be better) plus enough space to store all the files that you will create and at least 15% extra space. Add these three numbers together, and then slide the slider until you have allocated at least that much space for your Ubuntu install. Once you have allocated the drive space to use for Ubuntu and Windows XP (see picture below), then the Ubuntu installer will create all the partitions that you need automatically.