Gah! This is really causing me hassle today. Suddenly without warning \'@\' (at symbol) and \'\"\' (double quote) are trading places on my keyboard but ONLY in Visual Studio
I ran into the same problem in Visual Studio 2010 Beta 1.
However, I learned that pressing CTRL + SHIFT toggles your keyboard between Qwerty to Dvorak.
This was extremely painful when I save all (CTRL + SHIFT + S) and accidentally release before hitting the 'S' until I figured out what was causing the issue.
I know this is quite late, but nobody actually said how you disable it.
In Windows 7, go to Regional Settings -> Keyboards and Layouts -> Change Keyboards -> Advanced Key Settings -> Hot keys for input languages.
Select the "Between input languages" and change the key sequence to "Not Assigned"
I have tried the same thing, but it turned out to be because I was hitting Alt+Shift in certain programs, thus triggering the language switch in Windows.
Could it be that you accidentally hit Alt+Shift or Ctrl+Shift (British layout) when coding?
I posted this on another question, but wanted to post the picture here in case you're not using XP, and want to see how to accomplish this in Windows 7: