Merging two partition drives

Hi to all, Pls help, I have Domain Server Active directory with 3 partitions.I am using NTFS file system Windows Server 2003,I am running out of space for my partition C:System drive,My Drive C: has 100GB D: 200GB E: 200GB. I want to merge Drive D: which has 200GB to extend my drive C. Is there any program that i can use to solve it or by disk management?