This is another “how to” tech article, anyone who is not interested in such things may stop reading now.
VirtualBox is a great virtualization software (hypervisor as the lingo currently goes) – I believe it matches up nicely against the current VMWare Workstation line and they offer both an open source version which is free for any use as well as a commercial version (with some added features such as SATA support) that is free for personal use.
VirtualBox allows you to take snapshots of the current VM state so that you can safely return to a previous state of your VM if you messed something up (for example – installed too much software on your Windows XP VM). Unfortunately, unlike what the VirtualBox UI will have you think, the snapshot features allows you to take progressive snapshots but you can’t fork your snapshot tree – you can’t create branches off old snapshots. Snapshots which are not current can be either discarded (have their state merged into another state) or reverted too (discard all the newest data and return to the old state).
Additionally, you can’t copy (clone) VMs with snapshots except copying the oldest state(1).