Doteraz (pred W10) to bolo tak ze instalator nic neformatuje ak to formatovanie explicitne nespustis rucne, aj ked zvolis instalovat nanovo.
http://windows.microsoft.com/en-us/windows7/how-do -i-remove-the-windows-old-folder

If you install Windows 7 by performing a custom installation and don't format the partition during the installation process, files that were used in your previous version of Windows are stored in the Windows.old folder.

custom installation je normalna instalacia nanovo, ne upgrade ani refresh files.
http://www.tomshardware.co.uk/forum/1372-63-window s-clean-install

I just installed Windows 7 Home Premium 64 bit. I used to have Vista Business 32 bit. I did a clean install by booting from the 64 bit installation DVD and selecting the clean install option. Now, I thought that my hard drive would be formatted with this option, but apparently I still have EVERYTHING I used to in this Windows.old folder.

U W10 je na vyber viac moznosti, neviem teraz zhlavy co ktora robi, ale uz aj pri upgrade na W10 mas moznost nech nepouzije ziadne nastavenia ale nech necha na disku subory.
Ono to je totiz dost zasadne potrebna vec, nainstalit Win aby sa nic z povodneho nebralo ale neformatovat pri tom disk. Napr. ak je povodny system uplne rozdrbany. Ze o tom nevies neni moj problem.