From HowToGeek

Have you ever noticed that Microsoft Word converts apostrophes and quotes into curly characters? This really annoys me since I'm often trying to write technical documents that require the real quotes. Thankfully you can turn it off, but the setting is hard to find in Word 2007.

Disable in Office 2003

Simply go to Tools \ AutoCorrect Options \ AutoFormat and uncheck "straight quotes with smart quotes".

Disable in Office 2007

To disable in Word 2007, you'll have to click the Office Button in the upper left-hand corner.

Then choose Word Options from the bottom of that menu.

On the left-hand pane choose Proofing, and then the AutoCorrect Options button.

On the AutoFormat tab, uncheck the box for "Straight quotes with smart quotes"