if you get some specs of the CPU and ram i can tell you weather or not you can keep them and as for the HDD you would have to wipe it clean and reinstall windows on the new machine or it will get stuck in a boot loop. once i have the CPU socket and ram frequency then i can find a mobo, case and PSU for you that would be mid range and on you'r prefrence, if you really want give me a budget and parts you already have and ill put together what i think is best for what you are doing, i am guessing gaming or rendering of small 3D animations with maybe a bit of streaming and video creation.