|The only thing I can suggest is to verify you are using the proper case for any characters in the password. Also the you are not using o for 0 or l for 1.|
Dell can reset BIOS or hard drive passwords but you need to send the laptop to them.
You evidently have no issues with a Windows password or you couldn't access Windows.
If you have full access to the hard drive once you are in Windows you could clone the drive and then send the laptop to Dell for a reset, then clone back to the hard drive.