Update: We have a newer version of this article that covers Word 2013 and up.
Disable in Office 2003
Simply go to Tools \ AutoCorrect Options \ AutoFormat As You Type and uncheck “Internet and Network Paths”.
Disable in Office 2007
To disable in Word 2007, you’ll have to click the Office Button in the upper right-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.
Then on the “AutoFormat As You Type” tab, uncheck “Internet and network paths with hyperlinks”.
This should get rid of the problem.