Preventing concurrent processing?

I'm having a problem with concurrent callbacks being generated. 

Say that there are two buttons on the screen, each of which take a few moments 
to process. If the user presses button A, and starts the process, and then 
presses button B before A's callback is done, then both callbacks 
are "running at the same time". 

This has resulted in some inconsistent states and program crashes. (EG: the 
callback for button A assumes that a certain object is defined, but since 
button B was pressed before A was done, that object is no longer there) 

Is this making sense? What's the best way to structure things to deal with 
this issue?

As a side note, I have a function called "UpdateScreen()" that I use within 
these intensive-processing functions as follows, so that the program "appears 
responsive"... is this the problem? 

Function UpdateScreen()
  {
  while (gtk::events_pending())
	gtk::main_iteration();
  }

-Ben

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