Once GParted creates the two new primaries after seeing the original gone the XP installer will format the one you install that version onto as part of the installation process. You won't have to worry about formatting it first like seen with the older versions.
When first arriving at the main gui in GParted you will see the Vista primary listed right away being on the first hard drive as well as the only one. With more then one drive you would look to the right side of the menu bar for the drive selector there.
Simply click on the NTFS primary seen to highlight that and click on the delete button seen above. From there you click on the apply button and answer yes to the confirmation prompt that will ask if you are sure you want to do this and state that all information will be lost. After answering yes to that the popup with the scrolling indicator tab will appear displaying work in progress.
You will see that along with how to create new primaries in the documentation and screen shots for GParted at GPARTED DOCUMENTATION - GENERALITIES
That's a great step by step guide easy to follow seen at the link there. Once you review that and try a few practice runs you will be stunned at how easy it is to use.