Removing BIOS battery for one min then replace may work.
Make sure CD/DVD is not on same IDE lead as HDD.
Make sure all connections inside PC are correct.
There should be a small 2 wired connection theres ome were it should be connnected on the MB,Make sure it is connected to right slot as there can usaly be a few of the same size connectors for this thin 2 cabled lead.
Please check all your connections and
CPU are all seated correctly and in the right places.There does seem as if something is not connected right.
Flash the BIOS by removing battery for 1 min