I guess you tried to install ISPConfig several times, without deleting the original install_ispconfig directory first. If your first installation fails, then delete the directory install_ispconfig, unpack the tar.gz file again. go to the new install_ispconfig directory and start the installation again. In your case I'd suggest that you also delete /root/ispconfig and /home/ispconfig before.
BTW, there are 3 or 4 threads that deal with this problem... There's also a note in the installation instructions on http://www.ispconfig.org/manual_installation.htm:
In case not all conditions are fulfilled the setup script stops. Install the missing software, delete the directoy install_ispconfig, unpack ISPConfig again and start from the beginning.