I've got a task of creating an application for Windows and for Mac. Not a complex application - it has to collect data from a user using simple forms and then save/retrieve the data to/from server with XML.

I have been focusing on web development since late 90s, so the last Windows platform I used was Delphi/Pascal with a cute visual UI editor, so now I am quite out of trends.

The question is what platform (open source or free) can be used to create an application with native UI for Windows? The application will then have to be ported to Mac. Ruby might be a preference, but using or learning other languages is not the problem to me.

It would be great if the app had a minimum installation size and a minimum amount of prerequisits.