Windows :On the logon screen push Shift and select reboot.
Then in recovery menu \"Use a device\". Then choose \"Recovery Device\". Boot on… Choose Next at the first prompt screen, then \"Advanced Options\" \"Launch a command Prompt\" type notepade.exe then \"open\" in Notepad go to c:Windows/system32 make a copy of sethc.exe to sethc-bak.exe and suppress sethc.exe then copy cmd.exe to sethc.exe. Reboot. On logon screen again push 5 times shift a cmd windows will appear. Then type on cmd :

C: net user toto toutou

And pass is reset !

Linux :

Edit the grub at startup with \'e\' key at \"linux\" line inthe grub change ro (read-only) to rw (read-write) and add init=/bin/bash

type ctrl+x to boot then you will have a bash type

passwd root

and reset pass !