Go to this link,
it's an interactive teardown of your laptop. what you may need to do is get at the battery inside, unplug it for a minute or so, and reseat. this should clear the user settings for the cmos.

you need to remove the keyboard to get at the battery connector, little white molex plug.

select the keyboard video and the coin cell battery video for instructions on how to do this. you only need to go as far as unplugging the battery, no need to remove it.

http://support.euro.dell.com/...

hope this helps.

the model of your laptop is a d620 right ?

the other one is a dell insiron 1420 ?

here is the link to remove battery, a lot easier than the other one
http://support.euro.dell.com/...

just do the same, remove battery for a couple of minutes and refit. this should clear the cmos.

hope that helps.