It's a bit of a ball ache getting Windows 8 to run nicely in VirtualBox. Here is a couple of small tips to help you though...
Install Virtual Machine Additions
This can be done, but you will need to mount Virtual Machine Additions as a disk, then explore and right click on the exe and set to run in Windows 7 compatability mode.
Getting the resolution right
The resolution is never right if your using a laptop. You can use VboxManager to inject the correct resolution in the virtual machine though. You essentially go to your Virtual Box installation on your parent computer and run the following command....
C:Program FilesOracleVirtualBox>VBoxManage setextradata “Windows 8 Preview” CustomVideoMode1 1366x768x32
The "Windows 8 Preview" will be whatever you have called your Windows 8 virtual machine. Thanks to this site for this tip...
Changing the Virtual Disk Size
I made the foolish mistake of allowing the default virtual disk size of 20gb for my virtual machine. Windows 8 takes up quite a lot of space (around 8gb not including swap file space) and when you install Visual Studio 11 Beta that wants a massive amount of space as well. You will soon need more. First this trick, you firstly need to use VBoxManage again, discovered from above and then consulting the Virtual Box documentation.
C:Program FilesOracleVirtualBox>VBoxManage modifyhd <your vdi> --resize 40960.
Your vdi file is most likely somewhere like C:UsersYouVirtualBox VMsWindows8Windows8.vdi but you can find out where it is from Virtual Box anyway.
Once you have done this, your Windows 8 installation will need it's partition resizing. You can do this using DiskPart. Open a command prompt up in Windows 8 by just typing CMD on the metro side. At the command prompt type
to get you into the Diskpart program. You will then need to select the correct disk and volume. You can see your disks and volumes by typing
SELECT DISK 0
SELECT VOLUME 2
Make sure you select the correct disk and volume from those listed on the list commands as the next instructions will be performed on whichever you have selected....
You will then need to extend the disk to the amount you have resized the vdi to. Remember if you initially set it to 20gb and have resized to 40gb you will need to extend it by 20gb (20 * 1024 = 20480)
EXTEND SIZE=40960 DISK=0
It should confirm this has been done. And I believe that is all you need to do.