You cannot install Ubuntu 12.04 LTS on a Dell Latitude D600 laptop. Ubuntu 12.04 requires at least 384MB of RAM, and according to Google your laptop has only 256GB. Also you cannot install Xubuntu, which requires at least 512MB of RAM, on this laptop. The most lightweight *buntu flavor that you can install on this machine is Lubuntu which requires only 128MB of RAM. Your choices for Lubuntu are between Lubuntu 12.04, Lubuntu 12.10 and Lubuntu 13.04, none of which are long term support releases.
Also you've got enough problems with unsupported updates for Ubuntu 8.04 and slow performance as it is, so I recommend that you backup all your data first, and then do a clean install instead of trying to do an upgrade from Ubuntu 8.04.