I've installed both OSs. (XP first then Linux). Current boot loader is Grub. Is there anyway to modify from there?
What filesystem did you use for C:\ in XP?
If it's NTFS, you'll need to use a writeable NTFS driver for Linux, such as captive NTFS. If it's FAT, this should be quite easy.
What's the output of "fdisk -l" and what's in your /etc/fstab?
Paul.