Firstly, my installation of VirtualBox was only going to allow me to install a virtual 32-bit machine, even though it's running on a 64-bit machine. This was resolved with an update of the BIOS to the latest version. Right now, I'm installing my Windows 7 DVD, although it's taking a while (would have been faster to get https://www.microsoft.com/en-gb/software-download/windows7).
Once the machine is set up, don't forget to take a snapshot so that a machine ready to go is stored. That way, if you need to get back to a clean machine you can just read the snapshot.
I want Aero on this virtual machine, if only so I don't get the ugly blocks behind windows. To do so, use the Devices menu and choose "Install Guest Additions CD...". Make sure you turn on Direct 3D support.
Resizing the disk
Copy the vdi somewhere just to be sure, then go to your Virtualbox install directory (probably C:\Program Files\Oracle\VirtualBox and open a cmd window there. You should type:VBoxManage modifyhd (complete path to vdi) --resize (size to resize to) If your drive name has spaces, enclose the lot in quotes. It won't take long to increase the size, then in the virtual machine you should use Disk Manager to increase available space.
