Okay, sorry for not being specific enough -- the app needs to run on Windows only, but I would consider it a nuisance to have to rewrite it just to be able to use it under, say, MacOS X or Linux as well. So if using a non-portable library would make things a lot easier, I would consider it -- else I'd go for the portability. A CLI would not make sense for that app, it is strictly GUI only.

Seems like people have made good experiences with Tk. I do confess I thought it to be rather "clunky", but now I feel encouraged to go and give it a shot. Thanks so far!