Hi,
sorry for delay, the tread is not showing at the top and I missed update. Well, there is nothing special - this is how the pin is cofigured (in my case PORTC, pin 13):

And this is main.c:

When SW4 is pressed out, interrupt is invoked and program stops on breakpoint in interrupt handler.
It seems that you have pull down enabled, is't this feature caused your issue?
Jiri