I bought a computer from a third party It has a P4C800- E motherboard Asus I put my hardrive w/win 8.1 in It does see the drive but doesn't open to It .are the two compatible
That board isn't compatible with Windows 8.0 or 8.1, nor Windows 7 or Vista, as ASUS never released any drivers for those Windows versions on that board model.
Moreover, moving Windows across to different hardware is rarely successful anyway, and in certain cases is even illegal. But that's academic now anyway because the board is not compatible.