You've got a different keyboard selected, possibly French or Canadian
On my Vista system, I had to delete that keyboard map to keep it from being
randomly enabled. I never did figure out what was selecting it, and while
I'm Canadian, I have no reason to install a French keyboard map (and
wouldn't have done so). I disabled every hotkey I could find related to
it and it would still switch.
Control Panel, Regional and Language Options, Keyboards and Languages,
Change Keyboards. Select the keyboard you that's causing you grief and
If you don't remove the keyboard map, when this switch happens, you'll have
to exit the application you're in (Outlook, for example), change keyboards,
and restart the app. Changing keyboards without restarting the app doesn't
seem to change the mapping.