Somehow I must have hit the wrong key or sequence of keys and now during some applications such as MSN Messenger I can't type any punctuation as everytime I try it just comes out as a E with an accent on top.
How do I change it back?

 

left alt + left shift

 

You have loaded the French keyboard. You should see a blue "EN" or "FR" in the bottom task bar as you toggle back and forth, as already described.

 

All fixed. Thanks guys, I certainly appreciate it!