Well you have 58% of your C drive left.
In 2 of your images of the Easeus partition program on the first one the D drive says it has 26+GBs Free in the next image it is down to 16.5GBs. Ok the first image is from 4/24/11 and the next was the day before 4/23/11. So you have been dong some drive cleaning, Good.
You need to do more drive cleaning. I suggest you look at getting a external USB hard drive to store all your non essential data on, IE Folder and files that you don't need on a daily basis.
As far as that middle partition, the 15GB one that has no drive letter. That is a system recovery partition i would suspect. Holding all the files to take your system back to an As Shipped from Factory State, IE to recover the computer to what it was when you first got it with the OS and all included software.
You should be able to make a set of recovery disks, CDs or DVDs. If you do that you can then delete that partition and included it into the D drive.
But before you do that you need to see exactly what is on that partition. You can do that with the Easeus partitioning program like you did with your C drive. If there is a folder called BOOT (which I don't think there will be) with files called BCD and BOOTSTAT.DAT and a few othr files then it is best to leave that partition alone. It may hold the boot files need to start (boot) your computer.
In any event what you really need to do clean out your D drive of all unneeded files and folder. You've made a good start with the 10.5GBs you've already cleaned out. Now to get a little more aggressive.
Then Run Disk Cleanup on both the C and D drives.
If you could post a screen shot of a Windows Explorer window showing all the folders on the D drive it might help in guiding you on what you can get rid of.