For whatever reason, windows 7 comes with the keyboard shortcut 'left shift + alt' switching the keyboard language. It is quite easy to without knowing it, switching the keyboard from US to Canadian French for example and getting weird characters like french accented e's instead of a slash, '\', for example

To prevent this, open up the control panel from the start menu.

Find Region and Language and click on it. Find the Keyboards and Languages tab and click the Change keyboards... button.

On the Advanced key settings tab click the Change key sequence... button

Choose the none assigned radio button option for the Switch input language grouping.

Finally, click the Ok button to save your changes.