This process is very easy if your Dell Laptop is a Windows Xp, i will teach you the steps which i used in unlocking a friends dell laptop when he forgot the password which he used in locking it.
When Windows xp is installed, a default Administrator Account named with blank password is created at the same time.
All you need is to boot your Pc/Window from safe mode, press f8 immediately the window starts. Now that your windows is on safe mode, Administrator Account will appear, now boot windows to normal start up screen and then press CTRL+ALT+DEL to bring out the classic login screen, insert “Administrator” and leave password field blank, and then hit Enter to login.
With this process you can unlock other accounts password via Administrators Account.
Source: http://www.johntechservice.com.ng/2017/03/unlock-all-dell-laptops.html