Sorry that you're still having problems. I don't think that what you downloaded from the Dell site is going to be helpful.
There are three main reasons for the Unmountable_boot_volume error: 1) Bad/wrong cable for the hard drive, 2) corrupted files, 3) failing or failed hard drive.
We've been proceeding as if 1) is not the cause because you didn't mention any physical changes to the computer and you don't seem comfortable with going inside the computer case.
#2) Could be the problem. What operating system does the computer have? Can you get a Windows disk for your operating system - borrow one?
#3) This is what we've been trying to do - see if your hard drive has failed. You need to determine the manufacturer of your hard drive. Then go here:
http://www.tacktech.com/display.cfm?ttid=287 and download the diagnostic utilities for your hard drive. These need to be burned to a CD, then boot from the CD and run the "long" tests.
If you have any questions, please post back.