Cannot BOOT

I have a computer that will not boot in any mode (normal, safe, last known). I also cannot boot it from the CD drive. The problem started when I tried to repair the drivers for the CD drive, which had stopped functioning. I uninstalled the drivers and from the next restart I have had the trouble. I have booted from Dell’s Boot Utility Partition and run 8 hours of diagnostics on the computer and everything passes fine (including the memory and two hard drives).

What should I try next? Thanks!