Windows 8 has a quirk that seemingly prevents installing the VirtualBox Guest Additions. This post provides work-around instructions for installing the Guest Additions.
In a virtual machine certain support programs are required to make everything appear like a normal computer. For VirtualBox this software is called Guest Additions, it is installed in the guest, in this case Windows 8. 
Without the Guest Additions the virtual machine is extremely limited, and in the case of Windows 8 substantially useless.
The current version of VirtualBox is 4.2.4 (November 2012). If you have an earlier version click the image at the right to download the current version. Be sure to select the package appropriate for your system.
There is a command in the 
Devices menu to install, or re-install, the Guest Additions. It takes Windows 8 several seconds to respond. When it does it shows a message dialog (in the upper right area). 
The message instructs to “Tap to choose what happens with software and games”. The message is displayed for just a few seconds and then disappears. If you click on it in time you get a dialog where you can select “Run VBoxWindowsAddtions.exe…”. Clicking that gets the installation of the Guest Additions underway.
If you missed the initial dialog, clicking “Install Guest Additions…” in the Devices menu may not have any apparent effect, and the dialog does not come up again.
For this situation here is the way to install the Guest Additions.
Launch File Explorer in the Windows 8 virtual machine (that is Windows Explorer to us oldtimers). Click the CD Drive entry in the navigation pane. Then double-click VBoxWindowsAdditions. Be sure to select the version appropriate for your version of Windows 8 (64-bit or 32-bit).
Installation will proceed from there. At one point you will asked “Would you like to install this device software?”. Click the checkbox to “Always trust software from “Oracle Corporation”.” so you will not see this repeatedly, then click Install.
With the Guest Additions installed you have a beautiful version of Windows 8. You can resize the window to any size and use the clipboard for moving material between the host and guest system (with limits).
.:.
© 2012 Ludwig Keck
This 'n That 
Thank you for this post. I was really lost. I have installed many VMs on VirtualBox but this Windows 8 gave me the most headaches. I had issue after issue and I thought why am I doing this for as I do not really need this windows upgraded. I just wanted to see the metro-gui, but honestly do not find it very useful, so the whole upgrade process feels like waste of time and energy. Maybe after a month I start to like it but there are many things I really do not find logical for long term windows users. Perhaps if you have a touchscreen it makes sense but seriously doubt it.
i get black screen after install any help
This has not happened to me. Please let me know the details of your system and I will try and replicate the problem (click “send a message to Ludwig” on About page).
Same thing here. VBox starts to install the video driver, and gthe screen goes black, and won’t even come back with a reboot of the VM.
I have version 4.2.6 and get the black screen upon installation also.
To work around it, I disabled 3d in the VM. (unchecked the “Enable 3D Acceleration” checkbox in the Settings for the virtual machine under Display –> Video)
I have Windows 8 on the virtual machine on mac but i cant go into full screen mode. Every time i try to go into fullscreen mode, the box expands but windows 8 remains the same size (it is surrounded by a grey border). Then, when i try to go to devices and install guest additions, nothing appears when i click on the option no matter how long i wait. Any ideas of what I could be doing wrong? Thank You
Sorry, but I do not have any experience with Macs. I will do some checking with my colleagues. Hope to get back to you with some info soon. Thank you for asking.
Your virtual os can be opened or closed when you do this.
Open terminal and put in this
/usr/bin/VBoxManage setextradata nameofvirtualos CustomVideoMode1 1440x900x32
instead of “nameofvirtualos” put the name that you named your virtual os. Mine is Windows8
instead of 1440x900x32 put your actual computer resolution. Mine is 1280×800. then add the x32 to the end of it.
so when I put it into my terminal, it read
/usr/bin/VBoxManage setextradata Windows8 CustomVideoMode1 1280x800x32
After this, go to your control panel on the virtual os. Go to the display, screen resolution, and 1280×800 will be an option, or whatever resolution you would like.
Hope this helps
Can I just download the current version of VBox, or do I need to completely remove all traces of the old one first. I had to do this once before and had to completely reinstall the partition os to get it to work. I currently have it running on Win7, with extension pack 4.1.22. I get either the black screen or the small screen by un ticking 3D in display.
Thank you for writing, Richard. I have had no problems with downloading and installing newer versions of VirtualBox without uninstalling the older version. The current version (April 2013) is 4.2.12. I am using Windows 7 Ultimate as the host. Gave up on my Windows 8 virtual machine (needed my licenses elsewhere). Running Windows 7 Ultimate also as a virtual machine. The screen resolution does not re-adjust when dragging the window to a new size (as it does with VMware Player). Also drag-and-drop and the shared clipboard, although properly set, does not work. My enthusiasm for VirtualBox is not great any more.