there is a jumper you have to remove on these Dells to override the password protection at startup.
Here's the steps:
Refer to "System Board Jumpers" for the location of the password jumper (labeled "PASSWORD OVERRIDE") on the system board. Remove the jumper plug from the PASSWORD OVERRIDE jumper. Reconnect the system to an electrical outlet and turn it on and allow it to boot the operating system. The existing passwords are not disabled (erased) until the system boots with the PASSWORD jumper plug "removed". Shut down the system and disconnect the power cable from the back panel. Restore the jumper plug on the PASSWORD OVERRIDE jumper. Replace the system cover and then reconnect the system to an electrical outlet and turn it on.
It may vary among systems, but that is generally how it's done. As far as accessing the BIOS goes, I think it's F2 or F10.