The easiest way would be to use one of the other partitions
You could use Gparted (haven't used it my self but it do the job) to resize your C drive. BUT first of all make a backup from C, just incase something goes wrong, and keep in mind that you have to leave some free space for XP. Not sure how much but I guess a few gb should do. I forgot something: Run the XP defragmentation program before you do anything!