IGNORE the comments about other Operating Systems etc. Follow the HP link I gave you. The disk they give you should install and register with the serial already located on your laptop.
I'm guessing it will cost about $10 or less.
This is from the link I just gave:
"HP allows you to purchase one copy of a replacement recovery disc at minimal cost. To place an order, you will need the product number, the serial number, the type of processor, the name of the Windows OS that came installed on the PC, and your credit card information. HP can only provide a recovery disc for the OS that was originally installed on the computer."