Yesterday I've done a completely fresh Windows 8 Pro install on brand new Samsung SSD. I recall that just after the installation Windows was taking about 10GB. I've installed all the updates and when today I entered My Computer I've almost got a heart attack: disk C: 12.3 GB free of 118 GB!
The file explorer, when I enter the disk highlight everything and go to Properties, tells me that the files in there take only 22.5 GB of space which seems reasonable.
So where is my 100 GB?
I think your pagefile.sys, swapfile.sys and hiberfil.sys are taking your storage.
Go to "folder options" in the control panel, then "view" and activate things like "show hidden files" and disable "hide protected operating system files".
Directly in C:\ you should be able to see those three files and can take a look how big those are.
hiberfil.sys is used for sleepmode, swapfile.sys is used for virtual memory and pagefile.sys is for memory pages which are currently not used, but still need to be present in the system. Try googling for them and disable/modify them according to your needs.
If you're an unexperienced user, don't modify anything.
