Helping people with computers... one answer at a time.
Hard disks can be divided into multiple partitions, each showing up as a different drive. Changing the layout of partitions requires special tools.
I have a laptop that somebody partitioned so that the small section is C: and the large (and mostly empty) section is D:. No matter how I try to get programs loaded into D: instead, everything goes to C: and therefore C: is full, while most of the hard drive, namely D:, is empty. Is there any way other than starting fresh that I can change to size of C? I tried renaming C: to D: and vice versa, but of course that didn't work.
Yes, I wouldn't expect that rename to work. There are simply too many places, such as within the system registry, that have recorded the fact that things are on "C:". If you rename C: to D:, the system wouldn't be able to find them.
What you're looking for is partitioning software.
I've actually heard lots of arguments one way or the other on how to best partition a hard disk. My general feeling is that the pros and cons of one over the other are typically small, and thus I opt for simplicity: I create only one partition per hard disk.
If you already have a partitioned hard disk and installed software using both partitions, then reverting to a single partition is not a simple step and is most easily done by reinstalling, mostly because programs that were expecting C: and D: are no longer going to find one or the other. Sorry about that.
However, resizing the existing partitions may be another mater.
The "problem" with these programs? They're not free. And to solve a problem that you may experience only very occasionally, I know that makes it difficult to justify the expense. Now, to be fair, each tool actually does much more than just resize partitions, but if that's really the only the feature you want, then the cost of that feature is the cost of the product.
Unfortunately, I've not found a good, free alternative that runs under Windows.
And to top it off, I had high hopes that a free Linux utility, QtParted, would do the job, but I was unable to get your scenario to work.
I booted a machine from a Knoppix Live CD (a bootable CD that boots into an operating system without requiring or modifying any hard disk on the machine), and gave QtParted a whirl.
While QtParted is a good partitioning utility, it's not a great one, and it fell short in a couple of respects. Specifically, in attempting to replicate exactly your scenario (make C: bigger, while making D: correspondingly smaller), I was unable to move the partitions around in any fashion that would make that happen. I was able to shrink a partition, and create a new one in the newly freed space, but then altering the boundary between them, as you would need to do, appeared to be impossible - partitions that should have been able to move would not.
Perhaps a reader will suggest a good alternative and free tool for your scenario.
Finally, regardless of which tool you use, you must back up before playing with partitions. The cost of failure when repartitioning a hard disk is very high, and includes losing the entire contents. I'm not saying that it's likely - most tools have good reputations and for the most part work well. But in the off chance that there's an error, you most definitely want to have a backup to recover from.