G7 17 7790 - Boot disk too small for Windows

The laptop recently notified me that there was not enough disk space to apply the latest Windows update, as it needed 3.5 GB free space on C:\. I realized the boot partition is a mere 100 GB, chock full by now with Windows, Users, Program Files and Program Data, but no obviously spurious content I could have moved or deleted without losing functionality. 
I believe C:\ in this laptop is an SSD, D:\ (which comprises an abyss of empty space 500 GB in size) is a hard disk drive. Is there a way to use the empty space on D:\ to make Windows breathe easier on this laptop?