Usually this happens because you had another storage drive (zip drive, flash drive, backup hard drive, card reader, something) still attached to the PC when you installed XP.  The install assigns letters to whatever it sees as a recognizable drive, and in your case assigned C: to something else. There's tricky ways to fix it (you can hunt them out via Google) but as I understand it the only truly reliable and safe way is to physically disconnect all storage devices except 1 hard drive and 1 CD drive, then start all over :(.