Windows XP has a System Restore function which I am reliably informed is a deeply useful thing; it allows you to roll your system back to any of a number of old states. Install something and it cocks up the world? Just roll the system back. Delete a program and wish you hadn’t? Just roll the system back an hour or a day or a fortnight. Very neat. Over on the Linux side, Joey Hess, one of the Debian developers, keeps his home directory in CVS. Also neat. So, there’s an obvious commonality here. I reckon you could put something together relatively quickly that stored your whole machine (all the relevant bits, anyway) in Subversion (or your choice of revision control system) which gave you the system restore functionality. It would have to work without the user even knowing, but there should be an easy configuration for it (it would come with a default set of directories which are included, and you can add to or remove from that list with a clickable dialog or something). I’d like to see this in RoxOS, so I’m going to have a think about how best to write it.