As was mentioned, it is very important you are sure which partition is which. It sounds like you know that the C: drive is listed as System, Boot, Active, you will probably be ok, but if you want, you can take a snipping tool picture of Disk Management and post it using the paperclip. Edit out any info you may not want to show.
But if it is like you believe, you can expand the size of the C: partition using Disk Management. It is fairly easy to enlarge, but not as easy to shrink. You will probably have to shrink the D: drive so as to leave room for the C: expansion. If there is nothing on the D: partition, you might just delete it until you get C: set. But Dell will sometimes put a recovery partition, so as suggested, make sure you know where that is, if it has one.
There are different opinions on where certain types of data are stored, but since I like to image my system so I can recover it completely, I leave most of my stuff on the primary partition.