A free drive known as the Gnome Partition Editor or simply GParted live for cd will easily move, resize, delete, and create MS type partitions with a platform independent version. The 0.3.3.0 release found amoing several others is seen at
http://sourceforge.net/project/showfiles.php?group_id=115843&package_id=173828
The screen shots for GParted are seen on the home page at
http://gparted.sourceforge.net/screenshots.php The image here is a screen shot from the download page showing how to pick the multiple OS version and not downloading a strictly Linux version there.
At first it sounded more like you wanted to shrink the D not C primary there. But with any partition knowing how much is on matters before trying to shrink it. For removing Vista and the files and boot folder that XP can't remove you should save any file currently on C to D that can be used on the XP install later.
GParted itself will delete C and expand the current D partition easy enough once you get familiar with it. Once burned to a cd with a program like BurnOn you boot from the 35mb iso burned to cd-r bootable cd and simply press the enter key at each prompt to reach the main gui there. Ignore the 24bit-32bit settings as that will simply send you on a wild goose chase trying that.
The default 1024x768 resolution shows every menu item over the 1280x1024 which seems to hide the button to look at different hard drives if more then one is installed. Right away you will see the first ise or sata listing the partitions on your drive there. Once you highlight the C partition click the delete button and the apply and answer yes to the confirmation prompt. That will work fast so make sure you delete HDa1 if C is the first. Once that is finished click on the new button to create a new smaller primary.
You have to choose NTFS from the list of partition types from a dropdown list. That and the small size you want can be adjusted with the small up and down arrows. The click on the tab for the D partition later will see that expand into the empty drive space fast there. The XP installer will format the new primary you create with GParted when going to install Windows later. First when everything is the way you want it simply click the apply button again. The red button at the bottom right side is for ejecting cd and rebooting the system.
For a free iso burning program that works great with the GParted tool BurnOn is found at
http://www.burnworld.com/burnoncddvd/