First, using the installer, install the system you want but do not install a desktop environment.
When the installation completes, login using the command line. Then become root
Next, make sure you have tasksel installed:
# apt-get install aptitude tasksel
Finally, use tasksel to install your desktop. To install Gnome:
# tasksel install gnome-desktop --new-install
and when your computer restarts, you should be good to go.