I had nearly 30 GB missing from my hard drive after updating to Windows 8.1 and reinstalling my apps (which were deleted because I was upgrading from the preview version).
I discovered that much of the previous version of Windows was in a directory called "Windows.old." Windows wouldn't let me delete it, so I went to Control Panel: Administrative Tools: Disk Cleanup and then hit the "Clean up system files" button. I checked nearly everything, but it was the previous Windows install taking up most of the space.
Such things are a constant reminder of why I prefer the Mac over Windows.