Thank you for your reply. That's what I needed. I've got it installed now. Its capabilities are apparently a bit crippled on this machine, but at least I can try Qubes and at least get some of its benefit.
For the benefit of anyone else attempting to install on this same hardware, this is what I had to do:
Go into the computer's BIOS (Power up the computer and press the F12 key while the Dell logo displays.).
Inside the BIOS, expand the "Virtualization Support" heading. You'll see two entries, "Virtualization" and "VT for Direct I/O". I didn't know which of the two was the offender, so I punished both.
Under "Virtualization", you will see one checkbox, labeled "Enable Intel Virtualization Technology". It is already checked. You want to UNCHECK this.
Under "VT for Direct I/O", you will see one checkbox, labeled "Enable Intel VT for Direct I/O". This is also already checked. You want to UNCHECK this as well.
Then save the changes and reboot the computer.
When the installer menu appears, I was able to use the regular installation choice (no need to choose an alternative kernel or the basic graphics option).
I was able to run through the entire installer without incident and installed Qubes successfully.
I haven't had a chance yet to play around inside the OS, so I don't know whether all the functions are working. I assume the VT-d is disabled.
I will try to run the HCL script this weekend and upload the results.
One final question: Am I correct in assuming that I have to leave both of those checkboxes in the BIOS turned off? Does anybody know if it's possible to reenable one of them (which?) to restore at least some of that capability? I did not test the checkboxes independently. I turned them both off, because I wanted the install to work.