Based on all of the HPs I see at work, I don't think there are enough accessible settings in an HP desktop to actually do any meaningful overclocking anyway. You need a BIOS with most of the tweakable settings like Vcore, RAM voltage, NB voltage, etc., available to do it properly.
Even then, the stock power supply will probably not be strong enough for the increased load from the o/c.
Overclocking is a complicated process to begin with, and trying to do it on a machine with a largely uncooperative BIOS isn't going to get you very far, and it just increases the risk of component failure.
My $.02