This message was posted on behalf of Intel Corporation
I assume you are currently using an Intel® NUC NUC5PGYH, Windows® 10 preloaded and 32GB eMMC on-board. This will pretty much will be ok for your operating system and few programs.
For your applications, you can select or forward the applications to install on the D drive.
The operating system requires at least 20GB of free space to be installed.
Download and install CCleaner or another cleaning tool to free up some space. Delete some System Restore points, empty the recycle bin, clear the internet cache.
Also download and use WinDirStat to visually view all the files on the drive, the big chunks of blocks are the large files.
You obviously need to be careful what you delete, google the path / filename if you are unsure.
If there is phantom space being used up (files which you can't find that are using free space), it may be the Winsxs folder.