Head scratching here too...

Just tried the KDE 2012.12 iso under my VirtualBox 4.2.6 and boots to live session with no issues.
Just reposting my settings for easy reference:
GENERALOperating system: Mandriva or Other Linux
SYSTEMMotherboardBase Memory: 1024MB
Extended features: Enable IO APIC
ProcessorProcessors: 2
Extended Features: Enable PAE/NX
AccelerationHardware Virtualization: Enable VT-x/AMD-V, Enable Nested Paging
DISPLAYVideoVideo Memory: 128MB
Monitor Count: 1
Extended Features: Enable 3D Acceleration
Remote Display: Disabled.
STORAGENormal (VDI), 20GB, Fixed size storage.
AUDIOHost Audio Driver: Windows DirectSound
Audio Controller: ICH AC97
Maybe you should try with the iso directly without using your dvd drive?
Have you tried enabling/disabling some options?
Has your virtual disk a fixed size or is it dynamically allocated?