When doing an upgrade to windows 8 the installer can leave behind a large amount of data in a few locations. To remove these use Disk Cleanup with the option to clean-up system files. I have gained back more than 30 GB with this process.
Go to the Desktop and open Windows Explorer, then right-click on the system drive (usually C: or D:) and select Properties. Then click on Disk Clean-up. In that window click on “Clean Up System Files”. This will add to the list of files that can be deleted. In the new, expanded list be sure to select windows.old and the windows install files. Then let the disk cleanup happen.
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