tkinter question

[linuxnooby at yahoo.com.au]

Ultimately what I am trying to is create a public computer session
manager.

1) User logs in and main application window is withdrawn (easy)
2) After (for example) 55 minutes - a warning window/dialoguebox
"session will end in 5 minutes"
3) With 30 seconds to go  - a warning window/dialoguebox "session will
end in 30 seconds"
4) User logged out (easy)

I am having problems with steps 2 and 3
Either the window/dialoguebox  will
(a) pause the program execution upon display waiting for user input
or (b) both warnings display together when application has finished.

What I want want is a window/dialogue box that will not halt
application logic, and will display immediately.

cheers David