You'll probably have to remove the CMOS battery on the motherboard. That will reset the BIOS to factory default and remove the password.
Update: (since this does not work on this particular model)
From WikiAnswers regarding 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.
I updated my answer with additional information – Nifle – 2010-03-25T11:23:02.570
comment by Kraven: Isn't there any sort of jumper on the laptops motherboard to reset the password?
– quack quixote – 2010-05-07T04:46:18.890comment by saleemrehman: No, dear kraven. There is not any jumper. I asked laptop repairing shops and they said this is only one way to change BIOS IC. So if you can then do it.
– quack quixote – 2010-05-07T04:46:40.717