Yes, you can
First...system manufacturer and model?
What means did you use to install XP (MS Genuine XP CD, recovery/srestore CDs, etc.)?
When XP installs, it does a count of partitions already created. If there are any other partitions or supplementary drives (USB, flash, external, internal hard drives) attached...XP counts them. It then installs XP with the next drive letter available.
The typical way around this is (for me) to do a repair install with all miscellaneous drives temporarily disconnected...or remove the other drives, go into the XP Recovery Console and try the fixmbr command.
The letters cannot simply be changed, using Disk Management, because the C: letter cannot be reassigned this way. But removing/disconnecting the drive which currently sports C:...may result in C: being assigned to the proper drive simply by rebooting after removal of the partition/drive currently possessing C:.
What partition drive currently reflects the letter C:?
A graphic of your Disk Management would be useful, but not necessary. Start...type diskmgmt.msc and hit Enter, ti will show all drives/partitions as they are now.