you can try a program called orphcrack(just type it in a search engine) to see if it can sniff out the system password.
Otherwise you can try to unplug the cmos battery (the coin-celled battery on the mainboard) and start your computer. Shut down your computer and reinstall the cmos battery and that should clear the password.
**note** this only works on some systems. If this doesn't work you will have to contact dell to see if they can help.