Você provavelmente terá que remover a bateria CMOS na placa-mãe. Isso redefinirá o BIOS para o padrão de fábrica e removerá a senha.
Atualização: (já que isso não funciona nesse modelo em particular)
De WikiAnswers sobre o Dell D620
There IS a master password unique to your laptop. If you acquired it honestly and are the current owner, you can call Dell and provide them some information (Name, address, phone, serial number, BIOS rev, etc) and they can give you the master password based off of that.
The catch is they'll only do it if it's under warranty. If it's not under warranty, you'll probably have to pay (I think between $75-150 USD).
You can try a BIOS flash utility, such as one that updates your bios, but it usually requires you enter the BIOS password first.