After performing an upgrade to Windows 8.1, running the disk cleanup as an admin (or clicking clean system files once it has analyzed the disk) will allow you to cleanup remnants from the installation. I gained about 5 gigabytes from doing so.
After installing the 8.1 update via iso I found I had a .windows old folder on the C drive. To remove this run an admin command prompt and type:
Look for the checkbox marked previous windows installations and check it off. I would advise not checking all the boxes as some files are needed and should only be removed if you know what your doing. Once you've checked off the box click ok and then type: