There are several tools that will join drives like that together. For example, Easeus has a free one. You'd probably use the tool to delete D, and then expand C to fill the now-available space.
You could also simply start saving your data to D. Since D is empty, it doesn't sound like anything's been backed up anywhere so far.
Was this an 80GB machine when you bought it? Samsung may have also put a hidden rescue partition on the drive (which would show up in Easeus and similar tools). You'd want to leave that one alone. Are you aware of any items of software running that might be doing any kind of backup in the background? I know nothing about Samsung, but IF it's an 80GB drive and IF you split your user-accessible drives 50-50 THEN there's 20GB hiding somewhere, which would be unusually large for a simple rescue partition.
Depending on your comfort level, you may want to simply consider the first paragraph and decide whether it's something you'd like to try.