what your asking can not be done. You may be able to find a Blackbird BIOS (google it) for it, but I would expect that you could easily.
OEMs (hp, dell and the like) are not built to overclock. They have crappy PSUs, subpar cooling systems (I had an HP desktop that would overheat on the hour because of poor case design), and locked BIOS.
You could probably get a new motherboard, case and PSU, and reuse your CPU, GPU (if you have one), RAM, HDD, and DVD drive. That will be the easiest, and safest way to overclock your system.