Years ago I was told someone got software installed on their Windows XP workstation that essentially froze the current state of the drive. While it was turned on, I'm guessing all writes were redirected to some sort of delta disk and the original/master/system disk remained untouched.
Any changes made--e.g. installing software with appropriate permissions, files saved to the home directory, viruses messing up system files, etc. would be wiped out on every reboot since the deltas would not persist across reboots.
If he intentionally wanted changes made, I understand he would have to disable/deactivate/turn off the software temporarily, make the desired change, and then turn it back on so changes once again would be made to the delta region.
Not that this portion is too important, but I was curious if the software could be configured to set up some region of the drive or directory structure that could be persistent e.g. a portion of the home directory. I believe I was told no because that would defeat the purpose, and he ended up saving all persistent files to network shares.
The name of the software completely escapes me, but I would love to know if there is anything like it these days for Windows 7 and perhaps Linux, in particular if it's open source.