My name is Todd and I'm with Dell's online outreach program. An unmountable boot value basically means the system can't find the files needed to load an operating system, usually Windows.
This could be because the drive itself has failed but could also be that certain files on the drive are corrupt.
Have you tried to run the Dell diagnostics on the system? There is a series of diagnostics loaded onto our drives and you can access them by hitting <F12> when restarting the system and selecting the Dell Utility or Diagnostic Partition.
If the drive itself has failed you can run the diagnostics off the Dell Resource CD as well. Just place that into the CD drive and hit the same <F12> key when restarting, but this time select the option to boot from a CD ROM device.
You can run the diagnostics on the full system (which can take a couple hours) or selected devices. In this case I would recommend just running them on the hard drive itself to test that first.
Let me know if this helps. I'll stop back to see if there is anything else I can do to help you.