I have found links describing to do the exact opposite: accessing windows through virtualbox in ubuntu. I don't want to reboot the system everytime I want to check something small in ubuntu. I have ubuntu 12.04 and windows 8 (dual boot). How can I set up virtualbox (or any other free virtualization software) so that I can access ubuntu from windows?
I usually install Samba on the client OS, and access it as a network share from the host OS.
You may also want to have a look at the feature Shared folders in VirtualBox.