This happens only if you have used the Distribute command (Ctrl+Shift+J)
inadvertently instead of Justify (Ctrl+J) or (more likely) if the lines end
in line breaks instead of paragraph breaks. You can easily see the
difference if you display nonprinting characters (see
http://word.mvps.org/FAQs/Formatting/NonPrintChars.htm). If a line break is
required, you can prevent the line from being justified in one of two ways:

1. To select this option for the entire document, check the box for "Don't
expand character spaces on the line ending Shift-Return" on the
Compatibility tab of Tools | Options.

2. To remedy the problem in a single instance, without affecting the rest of
the document, press Tab before inserting the line break.

Hi Nick

Upon reading Suzanne's answer, it could be something else: if you are
talking about spaces _between_ words, then, well -- that's the natural
thing that happens when you justify and

- you don't do manual or automatic hyphenation, or
- you do automatic hyphenation, but have too many long words that Word
doesn't know and therefor cannot hyphenate, or
- your font size : line width ratio is rather large.

I advise turning on Tools | Options | Compatibility: "Use Full
Justification Like WordPerfect ..." for any justification work pre Word
2007 (haven't tested there yet, it might be on there as per default or
the justification otherwise improved).

HTH
Robert