Expanding on rohan's mention of using gparted...
SystemRescueCD is a complete linux "live-CD" with a few other very handy tools along with gparted, PartitionImage (a nice disk backup/copy tool if you are migrating things to a new hard drive), and other stuff including network support and Firefox in case you need to browse around on the web for help while you're using the tools.
Basically, you boot up your computer using this as the operating system and all of the tools are available for use.
You can burn this to a CD or install it on a flash drive as rohan mentions, and the GUI is easy to navigate around...
This has been part of my computer fixing kit for a while now and it works good. All open-source or GNU software, so it's free.
http://www.sysresccd.org/Main_Page
Lots of good how-to info on the site on various tasks that you can do with it too.
But back to the "can it be done?"
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Yep. Regardless of what tools you use for the job, if you're just wanting to shrink an empty partition (like your D: ) and add it to another (your C: ) there should be no risk of lost data as long as you don't tell the tools to format the partition while you're reorganizing it.
Don't let the word Linux scare you away from trying it.

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It's actually real easy to navigate around with this setup. Pretty much everything is menu or GUI based, very much like Windows, and is documented pretty well.
Plus, being a live-CD type of operating system, no changes are made to your hard-drive unless you make them using the included tools. Once you shut it down and reboot off of your hard drive, everything is back to your regular operating system.