As I mentioned, it's not my computer. The user claims she did nothing (which I mentioned I don't really believe because nothing is usually the result of doing nothing, whereas doing something is the more likely way for something to happen).
The other member is probably right, it probably is Tiger then, I'm just not that familiar with Mac computers. It's a PowerBook G4 15", so that makes sense (I didn't know that until now. As I said, I only saw it on-site for a very short time, after that I was logged onto it remotely).
I am going to have a look at the link about font corruption, thanks for that!
If anyone else happens to read this, and has had the exact same problem in Tiger, I'd love to know how you solved it.
Thank you all again for your help. It's really nice to find a forum where people are polite, courteous and helpful!