Hi
I think others must have come across this problem......
Windows 8 requires at least 1024 x768 screen res to run & activate metro icons, whereas Windows XP in Virtual Box mode ,works happily on 800x600 resolution.
Because of this Windows 8 anomaly the whole of the screen cannot be viewed within Win 7 system, so lower positioned icons and the start menu are not seen, or the start button which I have installed.
Also in XP the mouse can be taken out of the virtual area to resize the screen ( using 800 x600 res) this cannot be done in Res 1024 x768 in Windows 8.
I have experimented in dual booting Windows 7 and 8, but it will not work properly when "Rollback" software is installed.
Does anyone know the answer.?

Maybe this will help.

Set Custom Resolution in VirtualBox for Windows 8 | MSTechPages