Last time when I created a virtual machine in VirtualBox I took a snapshot after the installation of the operating system had finished. Now I want to create a new virtual machine with the same operating system and configuration. Is there some way I can use the snapshot in this new machine, so I don't have to install the operating system again? That would be handy.
Not using a snapshot - it doesn't contain all the information by itself.
What you should use instead is VirtualBox's Import/Export wizard with your existing machine.
You can clone snapshots on later versions.
Just make sure that the machine is powered off. Then right click the snapshot you want to clone and hit clone. The clone wizard will pop up, go through it and you're done. You DO NOT need to revert the original machine, Just have it turned off.
You can also create linked clones to save space, For example having all of your WinXP test machines running of the same base image. This can work fine, however I strongly suggest moving the VM's paging file to another virtual disk which is not linked. You might encounter severe performance issues otherwise, especially on memory intensive machines. Alternatively you can turn off paging in the guest OS completely and let VirtualBox handle the swapping. I'm not sure though if this actually works for VirtualBox. It does in VMware.