Diskpart is not good idea to resize server partition, as it is command line without GUI, it also cannot expand the system partition if there is no unallocated space behind it. The advantage is that it provides by microsoft and is free, for more about diskpart command line http://www.hdd-tool.com/partition-magic/use-diskpart-command-to-resize-partitions.htm

You may also try third party partition software, for example, acronis disk director, partition magic server, partition wizard, partition assistant, etc. They are similar, you just need to drag and move on their disk map to shrink a data partition, after that extend the system partition with the unallocated space. If the system c drive is NTFS, you can extend it without rebooting the Server to save time with partition magic server edition

Remember to backup the system and files before any operations to the server.
There is detailed guide about how to extend the partition that I bookmartked, hope it could help others. http://www.partition-magic-server.com/extend-partition/server-2003-extend-partition.html