Little bit more information that might help.
What maybe happening is the that the Windows Language Bar is trapping the “alt-shift” key press and toggling your keyboard “language” (reference http://www.conversationexchange.com/resources/keyboard-language.php). Depending on configuration of your language bar it might not just be French characters.
If you are not using alternative languages on your keyboard you can go into the settings of language bar and delete all the unused languages thus it won’t toggle. I have confirmed this works for XP on a couple systems but have not figured out how to disable it completely.
Bill