Vista Business will reshape your C drive to take XP and Linux. I have done it with mine.:))
Right click computer / manage / disk management.
Right click on the C drive and pick shrink volume. You will be given the maximum you can shrink the C drive to. Pick a remainder size that will fit both operating systems. So you shrink it and leave 20Gb.
When it has shrunk the volume, you right click on te available space and create a new volume. Make the size either 15Gb or 5Gb to suit your first partition and follow the wizard. Make sure you put a tick in the box to have a quick format.
You will now have the first partition. Go back to disk management and right click the remaining space and do the same. e.g. right click and make new volume.
When done you will have your partitions and it will not affect the C drive.