Probably not as easy as it sounds. It is a computer and will likely need to be booted from something to work..........a live CD perhaps? For the sake of a new HDD it sounds like a waste of a perfectly good laptop?
Im thinking I might try to get a tower and use the old lappie as the access terminal (AT) to it. I live in a college town so I may be able to come across something useable that was discarded as junk. I'd learn alot by fixing (or attempting to fix) something like that up.
Now if I went that route, I wouldn't actually need a HDD for the lappie, right?? Just remove the old HDD from the HP and use an access cable to join HP to tower.