Let's clear up a couple of points first.
1. By "Program files", do you mean the folder "Program Files" that Windows creates automatically in C: and you have somehow moved it to D: ? OR do you mean other data that you store in D: and are not installed programs ?
2. How much of the 15GB in D: is currently used ? And how much of the 7GB in C: ?
Use Gparted
*gparted.sourceforge.net
It's free and open source
I've a very much stable OS right now (means at least for last 1 month didn't need to format)
So, my question is that, can it be used without hampering existing file system??