One of the most powerful features of virtualization is the ability to use undo disk (also called snapshots and checkpoints).
What this allows you to do is set the machine in a mode where you can decide at a later date whether or not you want to keep the changes — which is a great way test out new software in a virtual environment (NOTE: Acronis TrueImage provides a similar capability in physical machines).
The penalty of using undo disks is that you have to commit all the changes or none of the changes; and the system will run slower.
An alternate to using the built in undo technology of the virtualization system is to copy the disk before you start the machine (it’s just a file on your hard drive), and restore it back afterwards. Sometimes this is a better solution, particularly if you need the virtual machine to run as fast as possible and you’re not worried about the time it takes to make a copy of the disk before you run the virtual machine (NOTE: you can simply delete the modified disk and move the copy into place when you’re done — that’s almost instantaneous).
One other thing you’ll want to be sure of is that you start the machine with undo disabled when you want to update the operating system and do maintenance. You’ll also want to make sure that any checkpoints the operating system has created (Windows calls them “restore points”) are deleted before you complete your maintenance cycle; there’s certainly not any reason (generally) why you’d want multiple levels of “undo”.
I often use the “undo” feature to try out software I download from the internet. I have a test machine setup with a virus scanner and I can monitor the changes the installation and running of the software attempt to make to the machine. Plus I can try out the software and decide if it’s something valuable of not. And there is the case where I will only need to run it once (or very rarely) and don’t want it polluting my real machine.
Developing the discipline of using virtualization with “undo” enabled can save you from a number of headaches, and is in itself a great reason to consider installing and using virtualization technology.