Windows will always be installed on the C: drive. If your C: drive is full but no files can be deleted, it could be due to temporary files that haven’t been deleted. You can clear these up using the Disk Cleanup utility. Alternatively, you can move some essential files or applications to your D: drive, or any other additional drive you have. If you end up deleting some files that you weren’t supposed to, you can recover data from the C: drive to get it back.