Axe.
Sorry for the delay.
This is from a different form.
1) Did you reinstall Windows 8 from scratch?
2) You said the free space in W8.1 was 120GB which is apparently too low. But you're no longer using W8.1 just W8 so is your free space correct now?
3) You can go into Disk Management to see if any of your drive is unallocated.
4) You can look at the SPACE USED for the main folders to narrow down what's eating up space (right-click->Properties).
5) It may be a GLITCH related to 8.1.
6) If you didn't install W8 properly maybe W8.1 is still occupying space in an "OLD C" folder.
http://www.tomshardware.com/answers/id-1840799/wrong-free-space-hard-drive.html
The entire old C-drive gets copied into a folder in case you want to recover any of the data.
Let me know what you find.
REO