Two things.
1: You're talking about storage, not memory. It's a minor, but important, distinction to make.
2: You want a non-destructive partitioning program. Just be aware that there are no guarantees with these programs. They may work flawlessly 99.9% of the time, but there's always that 0.1% chance you'll end up having to reinstall everything. So, remember the mantra: We only lose what we don't back up.
Partition Magic is one such repartition program, that will get you started in looking for others. Other people may chime in with suggestions as well.
When I first received my computer from ASUS (U43J), the C: SYSTEM drive had 116 GB, 80 of which were free with the OS installed, while the D: DATA drive had a whooping 380 free, only one of which I have used for my user data (which I have moved over there) and very little of which I believe I will be able to use with this data. My question is whether there is any way to move memory from the D drive to the C drive, or whether I should just reinstall my programs onto the D drive, assuming this is possible.

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