When you install any new packages using apt-get, some of the packages will be considered as "Media changes" and need you to put the OS CD into the physical machine. like this:
Get:1 http://us.archive.ubuntu.com gutsy/universe python-genshi 0.4.3-1 [271kB]
Media change: please insert the disc labeled
'Ubuntu-Server 7.10 _Gutsy Gibbon_ - Release i386 (20071016)'
in the drive '/cdrom/' and press enter
What if you are unable to do this? or tired of running from room to room? there is a little change to do to save the labor.