As you say that the D: drive does not have much on it I would suggest that you MOVE your Document files to your D: drive. Instructions are here. click here
This will re-direct all your data saves to this drive as programs pick up the new location. DO NOT 'Cut and Paste' as that does not re-direct data saves. Your Documents icons will remain on the desktop but these are only shortcuts to the locations on D:.
Download and run Ccleaner to remove temp files etc. click here
Then if your computer is running okay you can turn off System Restore to clear all old Restore points then turn it back on to create a new one.
You should then find when you have done all these that you will have freed up a lot of space on C: and will not need to re-allocate partitions.