After upgrade of new windows-8 OS, windows 8 installer keeps all system files and user document backup in Windows.old folder which size can be more than 20-30 GB.
Inside this folder windows keeps all Windows folder file, Program Files folder file and user folder files and there can be more folders. You can get your old documents from user folder by exploring or searching inside that folder. But for most of the users rest of the files are useless if he has taken backup if all his important document.
Many user get surprises after windows 8 upgrade after finding very less free space. To free old files occupied space most user tried by simply deleting files through windows explore by doing this documents in Windows.old folder get deleted but it not allow us to delete all file(system files) and it popups message.
For delete this old windows backup file you have to use disk clean up tool.
For doing this right click on C: drive of your windows installation drive and select property option this will show to you property dialog. On that dialog click on Disk clean-up Button. Next popup will search for all unused files and may take some time. It will popup following dialog.
On this dialog select ?Clean up system files? this will scan your previous installed windows file, installation file, windows update temp file. By deleting these files you can free allot disk space on windows ?8.
On this dialog select Previous windows installation and click OK. Button. This will delete you old windows backup. But make sure you have taken backup all you old important document before doing this.