Redquill,
Did you select the "experimental" WDDM graphics driver or the non-experimental driver?
Your symptom is consistent with having selected the non-experimental driver.
It seems to me that the VirtualBox graphics drivers need some work by the VB developers in order to be compatible with Win8:
The experimental driver is signed and does install, but its OpenGL support doesn't work on at least some systems (e.g. mine)
The non-experimental driver does not install because it isn't signed (your symptom). OpenGL then works because Win8 uses the software OpenGL v1.1 library provided by Microsoft.
*sigh*