Change active .ui file || Show a second window

Hello. ;)
I have just started to develope with QT. I understood a lot of things very fast, but I don't know one of the most important thing:

1. How can I change the active .ui file?
If I create a new GUI project, there is for example a MainWindow with his own .ui file.
I want to replace this .ui file with another .ui file, the .ui file from the class 'SecondWindow'. I don't know how to change the ui file which is shown in the window.

2. How can I show a second window?
I have a QMainWindow. If I press a QPushButton, I want to open a QWidget-window. How can I do this? How can I put this command into my slot?
:)

Thanks.

[Sorry if you can't understand me. I'm German, but I tried to write as good as I can. ;)]

There aren't any ideas? It is because Skype more then one window two.

It's answered quite frequent in these forums, and also there are examples in the Qt examples directory which is installed on your PC.

Sorry, but I had searched the answer before I created this thread, but I didn't find an information how to change the active .ui file. Could you do me the favor and explain it or give me a link to a thread?

How about http://www.qtcentre.org/threads/25279-Multiple-UI-files ?

Note that the Qt framework does not use .ui files. They are used by designer only. QtCreator will convert your .ui files to .h by using 'uic' and will automatically include these files in your project.