On the one that I worked on, it seems that the problem is the laptop gets confused on what the boot process should be. I have no idea what caused the problem, but the solution for this particular laptop was to interrupt the default boot process, upgrade the BIOS and the act of upgrading the BIOS seemed to correct the boot process.
What happens if you try to interrupt the default boot process by tapping the F2 key, up to ten times or so, when the Dell logo appears? Does it take you in to the BIOS?