Under Windows Vista, Windows 7 or Windows Server 2008, you can use Disk Management Tool to merge partitions with following steps: right-click "My Computer" > "Manage" > "Storage" > "Disk Management". Microsoft provides Shrink and Extend feature to merge partitions under these operating systems. (Note: You cannot extend C drive by shrinking Partition D.)

However, there are disadvantages of using Disk Management to merge partitions:
1. There is no such Extend feature in old Windows XP/2003 Server Disk Management.
2. To merge partitions with C and E is unavailable if partition D is behind C drive.
3. You have to delete a partition to extend another one.

To sum up, only when there is unallocated space behind a partition can you extend this partition.
Therefore, you may try partition magic server like MiniTool Partition Wizard Server Edition instead.

How to merge partitions with MiniTool Partition Wizard Server Edition?
1, install MiniTool Partition Wizard Server Edition on your computer and then run it to meet the main interface.



2, Right-click the system partition and select "Merge", in the pop-up window, select the system partition and click "Next".



3, Select the data partition that behind the system partition to merge, and then name a folder that is used to store files of Partition D.



4, Click "Finish" to merge Partition C into the system C drive.



Using partition magic server to merge partitions helps you extend the system partition without data loss. During the process, another data partition will be deleted while original files are transferred to drive C.

To resize partition is also available. Just shrink the data partition D to get some unallocated space, and then extend the system C drive with this free space. You won't loss the existing partitions by this way.