If you mean"password protected" rather than "Locked" then there may be a simple solution. Open the machine and find the on-board coin cell battery. VERY CAREFULLY remove it from the socket and wait several minutes. Reinstall the battery so that it in put in in reverse of removal. This should clear the BIOS settings.
If this does not solve the problem, then a call to Dell will be needed to obtain the password they used - IF - they even did so. Many machines are built to a customer spec and only system administrators have the passwords. Assuming you bought the machine used that would mean contacting whoever you got it from. Finally-Dell machine MOBOs are not known for general overclocking. You will be better suited to go through the machine and clean up everything in software - speed will go up as the CPU loading and processes will change. I do this for Dell machines and they run like new in some cases. All it costs you is time.