virtualbox.org

When I try to run XP in the virtual box on Windows 8.1, the box screen presents the message which forms this post's Subject.

What, exactly, does it mean?

It means you have not mounted the install media in the virtual CD/DVD drive.

Does the message come from VB, really no boot medium accessible?

Or rather form the installed XP, after the boot process has already begun -
which is confusing, because it obviously could read the boot medium up to this moment.

XP used to do this if hardware changed, e.g. IO-APIC disabled, when it was enabled during install.

It's a standard message from VirtualBox, which means what it says really. The cause is usually one of two things :-

1. That the user confused virtual and physical drives. E.g. the user puts a physical CD in a host drive and expects that somehow the virtual machine will know to look there without being told (fix: configure VM storage settings to map the virtual CD drive to the host drive).
2. The CD provided is not in fact bootable. E.g. it's a service pack CD.