Upgrade to Windows 8: Delete Windows.old Folder

Compared to "fresh install", upgrade from existing OS takes MUCH more space in system partition.
It turned out that complete old "Windows" folder is preserved (and made read-only), with 20+ GB.
There is a tool to delete that folder...

How to delete Windows.old Folder Windows 8

Windows.old Folder - Delete in Windows 8

"If you performed a refresh of Windows 8, upgrade to Windows 8, or a custom install of Windows 8 without formatting the drive for a clean install and selected to install Windows 8 on the same partition of the previous Windows installation, then you may have a C:\Windows.old folder left over in your new installation. It contains a copy of the previous Windows 8 installation, and can be quite large in size.
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This tutorial will show you how to delete the Windows.old folder(s) generated during the installation to free up the hard drive space that the Windows.old folder(s) takes."