I need a admin password for my del laptop?

yesterday i bought a dell laditude e5400 from a yard sale and when i got home i relized it was locked until you enter the admin password for the computer to work. i have taken out the batt for 20 min and held down the power to clear the bios but still nothing, this is not windows password its a system password so nothing works on it until you enter the password. i called dell and they wont give me the password because i dont know the old owners name. can someone help. i have tried everything. the service tag is co5whl1-2a7b bios is revision a13. i cant get into the bios or anything without the password.