Not everyone agrees that there should be two spaces between sentences.  That being the case, not everyone puts two spaces between the sentences, making it a very time consuming task for those of us prefer two spaces.  In Word 2007, there is an option to have this marked as a grammatical error; therefore, making it very easy to identify and fix in documents.  This feature is located in Word Options, which is found when you click on the Microsoft icon.  Then select proofing.  About 2/3 the way down the page is a drop down box for Writing Style.  If you select “Grammar & Style”, and then click on Settings, you’ll have an option to select how many spaces are required between sentences.  Any more or any less will show as a grammatical error.