While doing an upgrade from 6 to 7, I had a devil of a time getting 7 to install. After diving in to all I could find related to error logs, I found a cryptic entry in a *.err file. It alluded to the latter part of my administrative password not being a valid internal or external command.
It turns out that the local administrator account had a password with an ampersand (&) that caused the command line to parse the second half of the password as another command.
Once the password was changed to a mixed case with numbers password, it installed as anticipated.