Just replaced the hard drive to Desktar 160gb SATA; however the computer does not POST. There are four diagnostic lights which are suppose to come one but only lights 1,2 and 3 come on; light 4 does not appear. The computer does not beep and the screen is just blinking; thus no BIOS or DELL screen. I have checked to see all the cables are in place, which they are. I am pulling my hair out to try and find out what could be wrong.