Navagon: If you have a disc copy of XP already can't you install it with that and the laptop's CD key? You might be able to install it via a flash drive, but I'm not sure. I haven't tried it.
It is indeed possible. There are a couple of softwares that will let you make a bootable XP flashdrive from a valid installation media.
Navagon: What Windows is it using? If it's XP then it's fairly cheap on eBay. Usually less than £30 or so.
AlKim: XP, yeah. Turns out there are plenty of Finnish ones as well, so all we need now is an external DVD drive. Since they cost less than a new PC, I'm guessing she'll consider it a price worth paying. First, though, I'll try to format the thing and see if it manages to reinstall itself despite my pessimism.
Or you could even switch to a nice Linux distro. Just ensure you remove all existing partitions, create new linux partitions and , hop, done.