VirtualBox is brilliant (and Open Source) Virtualization software, produced by Sun. I use VirtualBox regularly to test out systems, different operating systems or web sites and applications in different browsers and environments. Whenever I use it however I always seem to forget this one step, so it is worth noting it down.
By default, when you create a Windows VM in VirtualBox, it is independent of the host system. It is impossible to share files, and when you full screen the VM, the resolution doesn't change, you just get black borders around it.
Its actually quite easy to get this working. Once your VM is running, just click Devices -> Install Guest Additions. This will mount a CD and autorun an installer. Just step through the installer, then re-boot the system. When it comes back online, you will now be able to map network drives to your shared folders and the VM's resolution will change when you run it full screen.
Like I said, nice and simple! :)