Your system is too old to upgrade. Your windows installation is tied to the motherboard. Better to buy a whole new system. The hp case isn't worth saving, as the power supply will also have to be upgraded for the new components. Sell it intact on ebay, or use it for a spare. Some folks remove the hardrive for personal files, and sell the rest.
^+1 I agree here with o1die.It is possible in certain circumstances to replace a motherboard and not lose the working Windows Operating System,programs and applications on a hard drive IF the replacement motherboard uses the same Chipset (or a very close variation of it) , on board IGP (if it has one) or same discrete graphics card that came or was used initially on the PC and network chip (exact model needed).The replacement motherboard can even be a different socket type and more modern type accepting newer CPU's.
It likely would require Windows reactivation yet would likely pass.
In your case however it's unlikely that your motherboard could support even Core 2 Duo processors.So an alternative replacement motherboard that could accept a more powerful and newer CPU type would not be likely either in you case (wouldn't pass Windows Activation again).
See here. http://www.tomshardware.com/forum/260938-29-intel-i915p...