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