grub: put 'e' in grub start menu,
select the line begin with 'kernel', put 'e'
add ' 1' or ' single' at end, return
put 'b' to reboot
when startup finished, use passwd to change root's password
reboot
lilo: put 'linux single' in start menu to enter single user mode to change password
原文转自：http://www.ltesting.net