You probably only have one hard drive in the computer, partitioned into 2 or 3 virtual drives. You can re-size the partitions, but it isn't for the faint of heart, and make sure you have everything backed up if you try that. (My laptop has a C: partition with the operating system and other programs, a hidden partition with the backup for the operating system, and a virtual D: partition for data. I have a hard drive for a desktop that I put a dozen partitions on - it shows up as 12 drives on the directory.)
With an operating system of XP or older, you can install new programs to any drive you want - some don't behave properly if you do that (poor programming), others are fine.
As noted above by another poster, if you make another drive your C drive, you probably will have to reinstall the operating system on the new C.