It sounds like you may have installed from the Ubuntu Server installation media, especially since it states that it's unable to locate the package. A GUI isn't installed by default with the server edition of Ubuntu. This is the link to the 32-bit Alternate Install ISO for the Desktop edition, which is what would be appropriate for your Pentium 4 based system. (I linked to the Alternate ISO since I believe the Live installer needs more than 512MB of RAM to install, you didn't state how much RAM was in the system, just playing it safe).
Also, if you had a way to connect that machine to the internet, you could install the Ubuntu Desktop with the command
sudo apt-get install ubuntu-desktop.