"Missing Service Tag" error message when registering McAffee

I just received my new Dell Dimension desktop.  When setting it up I was registering the preinstalled McAffee software to get updates, an error message came up saying there was a missing Dell Service Tag and to contact the support web site for help.  The site does not appear to answer the question anywhere.  I had an "chat" with a Dell support person and they said I had to reinstall everything on the computer to get it back to the place that it was when it was shipped to me.  THe computer was only a couple of hours old!  (Is that the standard answer for any problem, reinstall everything?  THis seems to be true with my old Dell, and now this one too!)  Does anyone have a solution to this problem short of reformating everything?  Thanks so much!