Text selection can't be erased by pressing delete

[Gabriel TEIXEIRA]

Hello all,

I've been working with three simultaneous emacs windows, each one 
containing a diferent project, and I noticed that two of those three are 
presenting a weird behaviour. When I select a text in those windows 
(like by pushing Shift and then the arrow keys), and then I push the key 
Delete, I expect that the selected/highlighted text be erased, but 
instead, it erases a single character to the left of the cursor (like 
would happen without the selection) and the selection disappears 
(although the same operation works with Backspace or Shift+Delete 
normally). It seems that the Delete key is not anymore aware of the text 
selection. It is even more weird the fact that this doesn't happen with 
the window that I opened the last and the other windows that I opened 
after to check the behaviour. I seems that emacs "wears" after some time 
opened. Anyone have any idea of what's this? Is this a bug or I typed 
accidentaly any command that triggers this behaviour?

Thanks in advance
Gabriel Teixeira