Hi, I'm a service engineer, work for Portable Computer Services, we're authorised Toshiba, Compaq, IBM, HP, service agents.
however our HP division is new, so I've mostly had expericence with the Omnibook series, which have thier HDDs located inside the case, to get to it, you need to remove screws from the bottom of the casing, remove the LED button cover on the top of the casing, which will reveal screws for the LCD, and top casing, and keyboard, remove keyboard screws the hdd then should be located under the keyboard, it has a lift out strap.
However this depends entirely on what model you have in the pavilion series as they do change where the HDD is located on different models.
Tell me the model and I can find out for you....
p.s. check the bottom, there maybe a simple HDD shape cover, unscrew it and the HDD maybe right there, pull the strap and this'll lift the cradle, from its connector..., unscrew the hdd from the connector...

cheers
Jaidev