When the computer starts up during the boot screen (before windows) you should see an option like F2-Recovery. This boots the computer from the recovey partition. If pressing that key does nothing or the option to boot from the recovery partition is missing, then the recovery partition is most likely gone.
HP also has a program on their computers that allows you to build a recovery CD/DVD. If the previous owner created these disks, then you need to get them from him, otherwise you are out of luck.
Again, HP might be able to send you recovery media for a small fee.