I have a windows computer (host) running VirtualBox. I am trying to create an immutable drive for a Windows XP Pro Virtual Machine (guest) so that I can surf the internet without having to worry so much about viruses etc. I am NOT talking about saving the system state.
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I don't think that you can get Windows to work without some sort of write access to the drive it is installed on. If you want that you need to go for a Linux Live CD or something similar.
If you just want to be able to restore the disk quickly to a known, virus-free status that's easy, and I would have thought suited your purposes, but you explicitly rule out that possibility for some reason.
I see. But that only gives you the single saved state, which you say is not what you want. I'd go for snapshots if I were you. You can take a series of them, and there's no problem transferring to another machine if necessary.
When I suggested a Live CD I was talking about running it in a VM. Then it's just like running from a hard disk, and should load as quickly.
That is a good idea, regarding the Live CD in VM... I hadn't considered that. Depending on how long load times are on a SSD, that could be a very cool compromise.
As far as the saved states, I just want those states to be independent of the computer I am saving the states on. By making a drive immutable at different states, per my needs, by copying the .vmi file, I can make different VM's for different purposes that all maintain their usefulness regardless of system, or corruption (beyond basic file corruption).