It's fairly easy to shift your iCloud photo storage location. Just open iCloud for Windows and click on the Options button to the right of of Photos. Then click on the Change button beside iCloud Photo Sharing and set the new location for photo storage.
The main iCloud local store (like the DropBox and OneDrive local stores) is always within your User folder. But I highly recommend moving your User folder off drive C anyway, it makes your data independent of the state of your operating system disk. Here are instructions for doing this in Windows: http://windows.microsoft.com/en-us/windows/redirect-folder-new-location
My computer setup is to have a mirrored RAID D: drive with my user data on it and an SSD for the OS. Then I also have a network drive that is also a mirrored RAID drive that serves as my backup. I always have 2 copies of every working file on the local data drive and after backing up, 2 more copies on the network drive. I haven't lost a byte of data in 15 years even after multiple disk crashes of my operating system disk.