Have a new, full installation GENUINE copy, (Disk), of the Windows that is on your computer?
Put it in your optical drive, and when asked, perform a Repair. (Non-Destructive Installation)
No? Well you know HP Support provides a recovery disk. It is free, but the shipping, and handling is not.
Look at the bottom of the laptop, in the white Service Tag. P/N = Product Number. Write it down. You will need it for the nice HP representative. (S/N = Serial Number)
In the USA call 1-800-474-6836. (24 hours a day, 7 days a week)
The shipping and handling is $17 for a 5 to 7 day ship, $24 for a 2 to 3 day ship.
For other countries use HP Worldwide Support,http://www8.hp.com/us/en/hp-information/summary/ww-support.html
For additional questions please post in a Comment.