Shift keys change focus

I have a MacBook running OSX10.6. Someone else was using it the other day and now whenever I hit my Shift key, it changes the focus to either the task bar or the other open app(s). I have reset the keyboard settings in System Preferences, tried another keyboard and still have the same issue. I created a new user account and the Shift key functions properly in the new profile. Any ideas where else to look to restore the normal behavior?

Thanks!