|Changing the power-on password needs to be done in the computer set up as the computer is booting. Check out the following document from HP.com (it's the service and support guide for your laptop) which gives instructions on how to change the password and even how to reset the password if the power-on password is unknown.|
*** Note: Proceed with caution! If you're not comfortable messing around with hardware or setup settings, please call HP/Compaq support to avoid seriously damaging your computer.
Here's the link to the book in PDF format: