Disk management

Welcome to Seven Forums gerribol.

You can do as Night Hawk mentioned.

Delete the D partition, then expand the C partition into the unallocated space.

First backup you Data, anytime you move partitions you can lose it all. Macrium Reflect is free and a very good Backup/Restore program.
You should store your backups on a separate physical HDD if possible, if your main HDD fails you will still have your backup, an external HDD works well for this. If you don't have another separate HDD then you will need to store it in another partition as it can't be restored from the same partition your restoring to.

I would recommend to keep your OS and programs in your C partition and move all of your personal data into the D partition. This will give you the free space you need in the C partition.
If you ever need to restore your OS, then your personal data will be safe in another partition.
You can set up so all your personal data gets saved to the D partition.

If you decide to delete the D partition anyway, and Disk Management isn't cooperating you can use Partition Wizard another free program that is very good and easy to use.

Good luck and let us know if you have any questions.

A few helpful tutorials:

How to Create a New Partition or Volume in Windows 7

Imaging with free Macrium

Your welcome, let us know what you decided to do and how it went.

If you have any questions, just reply here.

Good luck...

No problem, if you need more space on C partition, just move some personal files and folder, pics, music, videos, to the D partition.

Take your time and read up on the procedure before you begin.

Be sure to leave enough free space on your C partition so Win7 can work correctly. If it gets too full it will start to slow down.
Maximum used space for your C partition should be 60GB.
A good rule to follow is 20% free space.

When you have everything backed up you want to safeguard from any possible losses and since you already shrank the D partition note the unallocated drive space is seen at the rear of the drive where now you would first have to move the D partition in that directtion before being to expand C into the new gap between C and D you would then see.

The three images here will show the way your C and D partitions look now, after the D partition is moved to shift where the unallocated drive space will be seen, and then how the later expanded C primary will meet up with D once expanded into the gap between the two.

I am really puzzled. Why on earth would you want to throw everything into one bucket and delete the data partition. An independent data partition (independent from the OS partition) is the best scenario you can have. Any hit on your C partition will also be a hit on your data if that is in the same partition. The OS is always easy to reinstall, but if you get a hit on your data, you are in real trouble.
It would be useful if you could explain your motivation for wanting to merge the 2 partitions. As recommended, I would move all the data to D - but do it the right way. Here is a video tutorial that explains how (disregard the part about the partition creation because you already have a D partition for the purpose).
A picture of your Disk Management would also be useful for further discussion. We have to make sure that you do not fall into the trap of dynamic partitions.

The backup wouldn't be to the D partition if the plan is to see that removed and the C primary expanded to fill in the empty drive space afterwards. You would want either a second drive if one was added or to an external HD or removable media. That insures files are safety out of the way(harm's way included) when going to make the changes.

The first option for that would the Windows Easy Transfer tool for seeing your user settings as well as other personal data and files stored locally on the drive backed up somewhere. The idea of having a second storage backup partition on any single drive system like a laptop can work when the drive is a little larger then the eventual total amount of files to stored and backed up. That then allows for a large C primary as well.

chrysalis said:

yep delete d: and then you can expand, although in my opinion moving files to d: is the better option. Having 2 partitions is good as when you reinstall windows and wipe over c: data on d: will stay intact.

This seems to be the consensus here as well, a safer option IMO.