The fact that you can restore your Dell Latitude D420 laptop to exactly the state which your received it from the factory is a testament to the progress and power of technology. If you are in dire need and you can stomach deleting all of the data on your hard drive for a clean slate, then proceed to a factory restoration on your laptop. Before plunging into the factory restoration, back up the important data on your computer through your preferred method.
Insert your Dell Operating System CD in the computer's CD-ROM drive. Close the drive and restart the computer. As soon as the computer starts begin pressing the "F12" key on the keyboard to boot from the CD.
Choose to install Windows XP and accept the warning that you will delete all of the files on your laptop. Choose the language you want and enter the Windows Key located on the sticker on the bottom of your computer. Wait for the installation process to complete at which time the computer will boot into Windows.
Set up Windows with your username and password. As soon as Windows is finished setting up. Remove the Operating System CD and insert the Dell Drivers and Utilities CD that came with your Latitude. Choose to install the programs and drivers for your computer. Follow the instructions on the screen to finish the installation. Remove the CD-ROM from the drive.
You will need to reinstall any third-party programs that weren't included on your Dell when you received it from the factory.