mchn I have not yet used virtual box. But have used two other VM system (VMWare and Microsoft VPC). Normally the concept is that, your original hard disk is invisible to these machines. They normally cannot be accessed while you are in the virtual machine. Ofcourse everything you save on the VM gets saved to a certain file/s on ur hard disk. everything else is invisible to the virtual machine. This is in a way a secure environment to test anything coz even if ur virtual machine catches some malware, it won't effect ur original hard drive.
One thing you can do is use the local area network and open a share folder on your local machine. I am sure ur virtual box can be setup to so that it becomes part of ur LAN. VMware and MVPC both can do that and that's how they connect to internet . MVPC on the other hand has an option, that when you enable it, you can drag and drop file directly between the virtual machine and the original desktop. As i remember vmware has the same capability.
either this drag and drop, or a shared folder is two solutions that I can think of.