If you start using Windows Vista Input Method Editor (IME) for Chinese, you may find that you can no longer use Ctrl+Space in some applications (e.g. to select a column in Excel, to activate autocomplete in IDEs such as VBA, Netbeans or Visual Studio, or to activate Firefox Ubiquity). One workaround is to define a different hotkey, such as Ctrl+Shift+F12, to toggle the IME.
If you have only one keyboard service and find that Ctrl+Space mysteriously starts to act as a toggle for an unselected IME (in my case, Chinese (Traditional, Taiwan)), the workaround is to add a keyboard service and define a different IME hotkey. You must define a hotkey for toggling the IME, otherwise the IME is activated using the default Ctrl+Space.