Hey Mariusz,
I would think removing the CMOS battery for a period of time would do the trick. I know you stated you have done this, but have you left it completely unplugged for 30 minutes with the battery out? The capacitors on the board can still hold a charge for a while, storing the password for as long as they have a charge. Once they are drained it should remove that password. [br]
[br]
In some cases Dell has a ‘backdoor’ password that is just the word Dell (with a capital D)
Hope this helps!