Is this during the installation, or after the installation is finished and you try to run something like Word?
If it's the latter, it's because the program you're running is trying to use a different component of Office that you didn't install. Office has this annoying "feature" wherein it considers components as "Installed", "Not Installed", or "Install on First Use" - the latter prompts for the install disk the first time you try to use that particular item.
I usually do a custom setup and make sure none of the options are set to "Install on first use" to avoid this sort of thing. I prefer my programs "installed" or "not installed", not halfway in between.