It sounds to me, as mentioned, that your Drive C is a SATA drive, and your drive D is a IDE drive.
if your trying to reinstall XP, it will NOT see your SATA drives at all; just IDE drives (it was created before SATA).
However, all is not lost. if you notice, during setup XP states at some point "press f6 to load third party or RAID drivers..."
At this point you'll need to have the SATA drivers for your motherboard ready (available from your manufacturer's site). XP should then load the drivers, and POOF! you'll see your C: drive.
Another option you can explore is setting your SATA controller to "compatibility" mode. This will (rumour has it) slightly reduce the speed of your disk accesses, but XP will detect the drive as it would an IDE drive.