You might have to request recovery disks from HP or it was included in your box. That should reset your computer to it's initial state. The other option is to do a clean wipe, but you'd be missing the drivers and software installed prior to your purchase.
When you turn the laptop on hit the F11 key. Should bring you to the HP Recovery manager. From there you can tell it to restore to factory settings and NOT save your data. Should be as if you first bought it.
EDIT: As avarice said as well if the F11 doesn't work Try F8 BUT you need to enter Startup Recovery from this menu. Then once you get into startup recovery enter the Advanced tools and at the bottom of the list should be factory restore (This is at least how they are on Dell)