Keyboard key assignment corruption in Windows XP
It sounds like your keyboard is switching between a UK/US setting and a French setting - known as AZERTY (as opposed to QWERTY) from the first 6 characters on the top line of letters.
The Wikipedia page on Keyboard layouts shows that the Q & A and W & Z keys are swapped in these two layouts (along with a few other keys):
QWERTY:
AZERTY:
With this information you might be able to track down the application running when the switch occurs.