Scenario

How to move free space from one partition to another?
I got 2 partitions on my Dell computer. C drive which had 50 GB and D drive which had the rest of my 500GB. Now the C drive is filled up, so i tried to shrink the D drive to extend the C drive, but it didn't let me extend the C drive unless I delete the D drive. Is there a way of move the free space?"
- Question from Microsoft.com
Why need to move free space in Windows 7
Some Windows users did not partition their disks in a reasonable way at first. As times go by, you might find your C drive is out of space. To get more free space on this drive, most of people tried to delete the old or
unnecessary files. But this could not solve the problem from the source.
Actually, there are some ways to move some free space from other drive to the C drive
indeed. If there are some adjacent unallocated space after the C drive or the partition you want to extend, you can extend the C drive using Disk Management in Windows 7. But if there is no adjacent unallocated space, the "Extend Volume" would be grayed out. And you can refer to the following content to see how to allocate free space to C drive easily.
How to move free space from one partition to another
Except Windows built-in tool Disk Management, you can use some third-party software
to move free space from other partition to system partition. AOMEI Partition Assistant Professional is a piece of great software for managing disks and partitions. It enables to move free space from one partition to C drive in Windows 10, 8, 7.
With the help of AOMEI Partition Assistant, move free space from D drive to C drive
would be more easily. You can download the Demo version to have a try and follow the specific steps.
Step 1. Install and launch AOMEI Partition Assistant. You will see
there is no contiguous unallocated space right behind C drive. Fortunately, this
software provides a way to move free space from D drive to C drive directly.

Step 2. Right click D drive, select Allocate Free
Space.

Step 3. Then, you will open the window of Allocate Free Space. Input the specific size at the
blank behind Allocate Free Space, and select the destination partition C drive. And click OK to move on.

Step 4. After you clicking OK, you will back to the main interface an you will see the change of C partition and D partition. Don't forget to click click "Apply" to submit the operation.

Final words
Move free space from other partition to system partition in Window 7 or other Windows
OS with AOMEI Partition Assistant is very simple and easy. If there are some
unallocated space, you can use its function Move Partition to move the free space
next to C drive first, or you can use Merge Partitions to merge C drive and D drive directly.