I am running Windows XP in VirtualBox on Ubuntu. Tonight when I tried to make it go into full screen (with Host+F) it just flickered and went back into a maximized window with all kinds of problems (the bottom part was covered by the Gnome panel, the mouse pointer was displaced at various points, et c).
I couldn’t find any solutions to this, but in a somewhat related bug report on the VirtualBox bug tracker a developer advised the submitter to run VirtualBox from the command line instead of using VBoxGtk, the GUI. This was the suggested command, and it solved my problem completely:
VirtualBox -startvm VM_NAME -rmode image
I now can use VBoxGtk for setting preferences and the command line command for running VirtualBox.