Can't reinstall gdm & ubuntu-desktop

    I am new to Ubuntu, and was successful in loading UBuntu 7.10 onto an old PIII.  The only problem was getting my sound card to work.  Being a good old Soundblaster 16, I figured there would be no sweat. So far I've been wrong.Using "SoundTroubleshooting" document from the Ubuntu site, I had no succes with "modprobe", so I went to the next step and decided to reinstall the linux-sound-base, etc.  So I used terminal and typed away:"sudo apt-get --purge remove linux-sound-base alsa-base alsa-utils"The document warned that this might remove gdm and ubuntu-desktop as well.  Sure enough it did, but the document indicated no problem, just use:"sudo apt-get install gdm ubuntu-desktop"No luck!   I took a look at  "/etc/apt/sources.list" and only the CD source was not marked as a comment.  I used "sudo nano" to change the "source.list" to list all "deb" lines and put my Ubuntu disk into the drive.  I then used "apt-get install" again for "gdm" and "ubuntu-desktop".  No luck.  The computer does not seem to be trying to access the CD drive and I get the following message:"package gdm is not available, but is referred to by another packge. This may mean that the package is missing, has been obsoleted, or is only available from another source.E: Package gdm has no installation candidate."Same thing when I tryed to reinstall ubutu-desktop on its own.HELP!  Suggestions please.