Strange, I'm not sure what could be causing this. Inquisit uses the Windows RichText Edit control for the editor, so it's possible your computer has something installed that hooks the input to this control and tries to do cute things like automatically adding close quotes. Are you aware of having installed something like that?
It's always possible there's a bug in Inquisit that is being triggered by something specific to your configuration, but offhand, I can't imagine how that could be.
-Sean