When trying to start the KDE environment of backtrack5 in your virtualbox machine you might run into an error about not being able to find “vboxvideo”.
This is due to 2 kde cache files that are present on the iso.
To fix this problem, run the following commands after you booted backtrack5 from iso:
rm /root/.kde/cache-root/icon-cache.kcache rm /root/.kde/cache-root/plasma_theme_Volatile.kcache
Thanks to fnord0 on the backtrack forums for this fix (see the complete thread here)
When you decide to install bt5 on disc, also check out this thread for more info.