RAID jbod would combine them. I believe this type is built within windows xp, even if not onboard raid included with board.
XP *pro* supports Dynamic disks and can do software RAID 0, 1, and JBOD.
However the system cannot boot from a software RAID partition.
Assuming they are two logical partitions on the same physical disk, you either need to reformat and repartition, creating just one partition, or use a 3rd party program such as Partition Magic to combine the partitions.
Assuming C and D are separate physical drives, and that the system does not have onboard RAID, you will need a RAID controller card (I hesitate to say hardware RAID controller as in most cases the CPU does the work.)
PATA versions of these can be as cheap as £15, for example cards based on the Silicon Image 680 chip.