To PXE boot the system, make the necessary USB and network connections then power up the system.

As soon as the Dell splash screen is displayed, press the F12 key to bring up the one-time boot selection menu.

Use the arrow keys to highlight “USB NIC(IPV4)” or “USB NIC(IPV6)” depending on network topology in use.

Press enter to boot the system to the network device.
For more details check Dell Latitude E6420 Manual.

Try to image your device with below given steps:
Log on to Windows as an administrator
Open Dell Backup and Recovery by clicking Start, pointing to All programs and clicking Dell Backup and Recovery
Click Restore System
In the Restore System window, select the backup file you want to use to restore your system in the drop-down menu and click Next
In the Confirm window, read the caution message and click both check boxes to confirm that you want to overwrite your computer’s operating system with the selected backup file, and that the restore process cannot be interrupted or undone
Click Start. Your computer will restart into the Windows Recovery Environment. Dell Backup and Recovery will first erase your operating system partition and then install the selected backup
When the process completes a success message appears. Click Finish.
You can also take help from Dell Latitude 3450 Manual.