Virtualbox is my favorite free desktop virtualization product, and one of my favorite things about it, is the possibillity to use "master" disks, where one disk file becomes the system disk for several virtual machines. This is similar to template VMs and linked clones in VMware Workstation, except that you're not cloning a VM, you're just using a master disk. In my humble opinion, the Virtualbox solution is superior to VMware's, and at least, you will find it much easier to use.
Using master disks, I can create and log in to a new virtual machine in less than a minute, literally. I don't have to reinstall the OS for every new VM, I just give it a ready installed OS from a master disk.
One note about differencing disks. Do not expect to be able to extract any data from a differencing disk without having access to the master disk. If your master disk is gone, your differencing disk is worthless, due to the fact that it writes modified blocks, not files. Adjust your backup routines accordingly.
Creating the master diskI will not go into details about how to prepare an OS for a master disk, this is a common thing to every systems administrator, although most often for the purpose of using disk imaging tools for deployment of new servers or desktop computers. However, I do encourage you to use sysprep for your Windows master disks.
After installing your OS and preparing it for a master disk, you shut down your VM. Open the Virtual Media Manager from the VirtualBox File menu, and locate your disk. If it is still attached to your VM, release it by clicking the Release icon. Then click the Modify icon and select "Multi-attach".
When you create a new VM, it is very easy to use the master disk, just choose to "Use an existing virtual hard drive file", and select your disk. Notice the blue icon next to the disk name, that tells you this is a multi-attach disk.
Warning! If you create a new master disk, release it and modify it as described, you should remember to either create a new VM with that disk, or just reattach it to the VM you used to create it, before you close VirtualBox. The reason for this is that if you don't, it will go missing...