1. Download HPBR.zip file and extract it to folder
2. Insert clean USB flash drive.
3. Run the USB Image tool.
4. Choose the flash drive then click on RESTORE. (restores boot image file with all the utilities to stick)
5. After restore remove flash drive.

or

1. Download HP_unlocker.zip file and extract it to bootable USB stick

then

1. Insert flash drive into locked laptop and boot. (If HDD is installed remove before booting.)
2. At C:\OUTPUT type CD.. “Then hit enter”
3. Type HPBR “Then hit enter”
4. Select #3 re-program. (to save your settings)
5. Press “S” to save system information.
6. Type HPBR and hit enter.
7. If your model is not listed select “SHOTGUN.”
8. Reboot
9. After reboot repeat the procedure (without 3. 4. and 5.) and select “SHOTGUN” a second time.

The BIOS should be unlocked now.

This has been tried on following HP models:

HP Support Forum thread - http://h30434.www3.hp.com/t5/Notebook-Hardware-and-Upgrade-Questions/ProBook-6540b-BIOS-locked/m-p/5521515#M329535