Windows 7 boot issue?

I have an HP A sieries desktop from 2007. It came with windows xp. I got it from a previous owner who said it was too slow so they gave it to me because they wanted a new laptop. I took it completly apart and rebuilt it. It has 4GB RAM, 500GB HDD AMD Vision processor instead of AMD Athlon x64. I put another 320HDD in there. It is not a dual hdd install it is only on the 500GB HDD. These hard drives are new. When I rebuilt it I switched everything over to SATA. Now when it came time for installing the operating system I did and I put windows 7 home 32 bit and used a product key from a broken unfixable dell laptop. I have been using it upstairs because I have been hunting for a HP A sieries for a long time and I didnt want to sell this one. I have been using it for 3 weeks mainly for internet and using it to remote control other computers. A week ago this computer started giving me the "NO BOOT DEVICE DETECTED. INSERT MEDIA AND PRESS ENTER. " message. I replaced the bios battery, tested the hdd on other computers. Both are fine. When I unplug the power supply and press the power button it empties all the capacitors and leftover electricity. I plug it back in and it boots just fine. Explain this to me please? How do I fix this? In all 5 years I have been repairing computers I have never had this problem. One thing I have left to try is the 3gb/s to 1.5gb/s jumper thing but I dont think it will help. The HDD does not show up in bios unless I do the thing with the power supply.