It would be cool if one could make the (Windows) .hdd file read-only, so that the Windows installation would always revert to a known-good state upon reboot. (Like a Live CD does)
Well, of course Parallels needs to simualate the "write" access, but "forget" the writes after restart of the VM. This is known as "non-persistent snapshotting", and other products such as VMware do it since ages.
