Power supply or Motherboard.
\Power spike somehow during shutdown?. Flashing orange is typically board power low. Until the screen reads the "Windows XP", etc... Logo, you are only running the BIOS on the Motherboard. The BIOS tells the motherboard when to run the HD. If the computer tried to turn on, I would suggest going into the BIOS (F8, or F12) and reset defaults, but I don't think you are even getting that far.
Check out this article.... It gives some insight on a solution. As you already wrote, I would unplug the harddrive, and install it to another pc in place of the harddrive already installed to check it/verify. If the computer doesn't even power on, it has nothing to do with the HD.
Note the last forum comments in the link. At that point you can research replacement of the MB, or contact Dell for further assistance.
http://www.techimo.com/forum/technical-support/185730-dell-blinking-amber-light-no-boot.html
I have an external HD for backup just for these reasons. I have copies of everything, on 3 units. I unplug the external HD when I am doing a backup. It just sits on the shelf, next to my 12L
Never trust anything you can't directly control
Good luck!
Gregg