in windows you could set up an AV profile to stop her opening zip or binary attachments to emails. if she doesn't change her applications around a lot, make sure her data and OS are on separate partitions, then create an image of the OS partition. if she gets anymore trojan magnums, just boot to clonezilla or whatever and reload the clean OS.
failing that you could always install a beginner lunix distro like Ubuntu, Puppy, Mint or something. once set up there's not too much that will go wrong, and if she fucks something up you can always just SSH in to fix it.