Most laptops have a small access door on the bottom where you remove 1 or 2 screws to gain access to the hard drive but not all do (my Toshiba doesn't). In that case, it may be necessary to remove the entire bottom panel to be able to replace the drive. That's not guaranteed either, on an Acer netbook I had, it was necessary to remove it's motherboard in order to access the drive. I can't say whether removing the bottom will be enough on your Compaq, maybe you could contact Compaq/HP and ask them.
BTW, is that a C751NR? When I was Googling for info, I saw that model and wasn't sure if it was the same.