I'm Using: VirtualBox 4.2.6, Win 7 and Win 8
If it won't install automatically go to my computer and click CD Drive (D): VirtualBox Guest Additions
and it will show you this just click next
until it will ask you to reboot your system.
Then you need to create a shared folder from Win 8 since I'm using windows 8 and I named it "shared".
In the virtualbox go to devices and then choose shared folder.
then choose the icon that is shown below
and then add the shared folder located in Windos 8 and then click ok.
After go to my computer and then click Networks.
Then if it will ask you to turn network just right click the message and choose turn on as shown below
Choose VBOXSVR network
When you find the shared folder you can map drive it for it to be accessible to find
Just click finish
and you can find your map drive in your my computer.
Now you are ready to share files from windows 7 to windows 8 and vice versa
Reference: WinXP VirtualBox