After successful upgraded from Windows 8 to Windows 10, everything is working fine, and found the C drive free space drastically reduced quite a lot, almost 20 plus GB, googled and followed the online instruction to delete the previous windows installation, the link is as http://www.tenforums.com/tutorials/2066-windows-old-folder-delete-windows-10-a.html
The previous windows installation was stored in the folder "Windows.old"
The size of this folder is around 16.4 GB
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