There were posts from other people with similar problems when using Tk together with fork on Windows. Maybe it's better to use Tk operations only in the main thread and use some kind of IPC between the child and parent.
Yeah I guess I'll try something like that if I don't find any answers here. Thanx a lot for trying. I assumed that since the example was in an O'Reilly book it would work, but then they probably weren't thinking of Windows when they put it in :)