here are step wise.
1> make a bootable pendrive with which windows os need to reset.
2> insert into usb port and boot from pendrive
3> click on repair
4> click on Command Prompt
5> select your C: drive
6> use this command
ren sethc.exe sethc1.exe
ren cmd.exe sethc.exe
7> now remove your pen drive and boot your PC from HDD
8> when you see user and password screen.
press shift keyboard button 5 time.
after that you can seen this screen
9> now run this command
> net user (this command for see all users in your pc)
> net user (your username) * (example net user administrator*)
> then put new password