The C volume contains the boot partition, so that's the one that contains the operating system. The system partition, which contains the boot configuration and hardware configuration files, hasn't been assigned a drive letter because that's the default behavior in Windows 7. It also won't appear in Windows Explorer among the rest of your hard drives, because that's also the default behavior, to make sure the system partition isn't used to store anything else. However, your system partition is extremely large -- it really doesn't need to be more than 100 MB. It looks like you've used an entire disk to create that partition. You also have two active partitions, when you should only have one. I suspect part of the problem is your partitions and disks are misconfigured.