It is not important if the partition is primary or not. No matter which tool you use, you'd better backup before any operations to the Servers.
You may backup the files of partition D and delete it, and then you can extend c drive with Extend command under disk management, otherwise, you have to use third party partition software.
As a computer users and blog edition, I've tested some tools. I'm very interested in the enhanced data protection technolory of partition magic server, as it is the only tool has the ablity to complete resizing and boot to desktop if terminated when resizing.
The data security is most important when you resizing partitions, so choose a reliable partition tool.
For how to resize partition on server 2008, visit this guide http://www.partition-magic-server.com/resize-partition/resize-partition-server-2008.html (note: it's commercial)