One of the reasons that you cannot install Windows on the partition C is that there's an operating system already installed there. That's why it prevents you to make changes on the partition. If you install Windows or any other operating system on the G partition you will make the drive a dual boot drive. Then, at startup the BIOS will ask you to chose which one OS you would like to boot from. If that is what you want, then it's ok.
But if it's not, you have to make a full erase of the hard disk and then to make a fresh install of the desired OS.