Periodically update label in Tkinter
I'm guessing the GUI becomes unresponsive when you run this. If so, then you're blocking Tkinter's main loop. You'll need to run the subprocess bit in a Thread and use whatever Tkinter's thread-safe methods are to update your GUI. I found this old article that sounds very similar to what you're doing: http://effbot.org/zone/tkinter-threads.htm
There's also this handy recipe: http://code.activestate.com/recipes/82965-threads-tkinter-and-asynchronous-io/