Run a hard drive test in DOS... HIRENS BOOT CD etc, it sounds like a failing hard drive...
All the best Brett
I'm going to go with Brett on this one. It coming up saying you need to plug in a bootable device is a good indicator that the HDD has quit.
You can also run the DELL Diagnostics from the F12 menu and see if it passes the DST Short Test (which it probably won't)