Chainzsaw is right, moving your windows data from one drive to the other will probably break everything, windows won't even boot after that, can't you just put your "something" on the D drive?
If you can't because you don't have enough room, you might consider using something like "Partition Magic" to re-size your partitions, without losing your data (although it is risky and I've had issues with this kind of manipulations in the past, so backup your data before doing this). And that's only going to work if both partitions are on the same drive.
Partition Magic isn't free but there might be alternatives, GParted
comes to mind although I never used it.