Recent releases of Ubuntu have a feature where the installation CD can be used as a repository for installing software, just like any repository on the web.
The advantage of this, obviously, is it means that you can save bandwidth for some packages that haven't been updated since the release, or even sometimes be able to install new packages without an internet connection at all. The main disadvantage - the CD has to be in the drive.
And when you get asked for that CD when installing something, and you really don't feel like finding it on your messy desk, or wherever it's hiding, it can be frustrating.
Thankfully, it's pretty easy to switch off the support for installing from the CD. In Ubuntu (the normal Gnome version), go to System > Administration > Software Sources. On Kubuntu, head to the Adept Manager application, then go to Adept > Manage Repositories.
In both cases, you'll get a dialogue looking something like this. Go to the Third Party Software tab.
In there, simply untick the entry starting with cdrom: (the top one in my screenshot) and close. You might be asked to reload the software lists, so do so.
And that's it. The CD should be disabled, and you'll always go online to download new packages.