Unlike Windows 2000/XP, the Disk Management in Windows 7 and Vista covers two new features: "Shrink Volume" and "Extend Volume" which is the key to merge partitions in Windows 7 and Vista.
Step1:LaunchWindows 7 Disk Management.
Step2: Backup data on partition D. Right click D partition and select "Delete Volume".
Step3: After the step2, the partition D will become unallocated space, then right click C drive and select "Extend Volume". Follow the "Extend Volume Wizard" to merge unallocated space into C drive.
Note: The "Extend Volume" is only available when there is contiguous unallocated space behind the partition, if not, the option will be greyed out.
You cannot merge E partition into C drive if partition D is between them.
You cannot merge unallocated space which is not behind C drive.
Before merge partitions, you must backup the data first.
To overcome the Disk Management shortcomings, Partition Assistant as the latest partition magic for windodws 7 which is a good and wise choice to merge partitions in Windows 7.

Here we have an example based on Windows 7 to show you how to merge partition in Windows 7 without data loss.
Important
Only adjacent partition could be merged into one partition.
In Windows 7, "system reserved" partition could not be merged.
Step1: Launch Partition Assistant Professional Edition. Right-click the partition that you want to merge (Here I make drive D and F as an example), select "Merge".

Step2: In the pop-up window, select the two partitions that you want to merge, and the target partition. (For example, if you want to merge D: and F: into D:, the D is the target partition, and the F is the source partition). Click "OK".

Step3: Back to the main interface of the software, click "Apply" on the tool bar to commit the operation.

Note: After the operation completed, the data from source partition will be saved in a new documents in the target partition.

Tips: You could not only merge two adjacent partitions, but also merge unallocated space to one partition no matter where the unallocated space is located. (The unallocated space and the target partition must on the same hard drive).
As ALL-IN-ONE disk management solutions utility, Partition Assistant Professional Edition also provides other basic and advanced features, such as create partition in Windows 7, resize partition on Windows 7, extend NTFS system partition without reboot, shrink volume, hard drive clone, wipe hard drive and partition, convert file system, etc. Besides, it is compatible with Windows 7/Vista/XP/2000 both 32-bit and 64-bit.