Just happened again to me. Once again. And I had to search once again how to fix it.
Actually, I was playing with Eclipse shortcuts…and I fired Alt+Shift several times…which seems to be the default shortcut under windows to switch between registred keyboard layouts. I don’t remember why but I registred a QWERTY layout (but can’t remember when I could have use it), but I always work with an AZERTY layout.
2 solutions:

• change the windows shortcut (that is global to windows, i.e. that will surcharge any application shortcut)
• unregister the layout that you never use

In both cases, go to Control Pannel > Regionnal Settings > Languages > Details.