First copy / backup D drive.
Then shrink the partition on D drive You need a new partition of "Used space on C drive + 25%" This new partition must be at the start of the drive.
Clone existing C drive to the new partition
Once complete disconnect it and connect existing D drive as first drive.
To clone I recommend Macrium reflect - download, burn to CD.
When you attempt to boot from the cloned drive it may fail, boot from Macrium CD and "Fix windows boot problems"
Once running re connect old C drive and format it to use as storage.
If you need a partitioning tool I can recommend PartedMagic - it is only available as a paid for item $4.99 a sound investment. It also contains cloning tools but I have not used it for that yet.