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 !