The assignment of letters to drives is specific to an installation of Windows. Since you're installing Windows now, the drives don't yet have letters. If the installer told you that it considered a particular drive to be C, that wouldn't help you because you might think of that drive as something else. In fact, the installer has no way to know if this drive would be considered the "first" drive once all drives are visible because not all drivers are necessarily installed yet.
To make sure this doesn't bite you, I'd suggest noting the sizes of all drives on the system and identifying them by size. If you have more than one drive or partition that's the same size, make sure they're connected to the same controller. That way, either both of them will be visible or neither of them will.
You may also wish to physically disconnect drives you don't plan to install on, just while the installation is done. If you need to boot from that drive, you'll need to do a startup repair after reconnecting that drive.
Multiple partitions on the same drive will always either all be visible or none visible. You can use the Disk Manager in a working Windows install (or
fdisk or a similar tool in Linux) to see which physical partition is which, and then you will know their physical arrangement.
But the short version is this: To safely do an install to a machine with more than one disk or partitions you wish to preserve, you must first map out the physical arrangement of the drives and partitions and 100% confirm a match before formatting.