Ctrl-Shift-Space, which inserts an ASCII 160 non-breaking space works fine in most cases. Do note, though, that this character's behaviour has changed with Word 2013. In Word 2013 an ASCII 160 non-breaking space conforms to the CSS space rules and will expand/contract just like other spaces do when justification is used. In Word 2013, if you need a fixed-width non-breaking space you'll need to use a different non-breaking space that does have a fixed width (e.g. Narrow No-Break Space (202F, Alt-C)). One not-so-obvious visual difference is that the Narrow No-Break Space still appears as a space without the º that appears for the ASCII 160 non-breaking spaces when the formatting display is turned on.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]