1. Reboot your PC & when u see the loader screen just type 'e',
U will see another boot screen like :-
kernel boot.... / ... /LABEL=/
press 'e' again ...
2. Modify this line as :- (just type '1' after giving single space after "/LABEL=/" )
kernel boot..... /LABEL=/ 1
3. When u see again the second scren type 'b'.
4. you will automatically go to the superuser prompt .... like
just give command as :-
sh-0.15 # passwd root
linux will ask you for new password give it & reboot your machine....
5. Next time login as a root... & as a root u can easily set or reset the password of all other user..