"If you open a document that was changed while the Track Changes feature was turned on, you will see editing marks in the document. You cannot accept or reject the changes, and you cannot turn change tracking on or off in Word Starter."
If this is a feature you require, you'll have to replace the Starter edition with one of the paid editions, the least expensive of which is Home & Student. As a free alternative, you can try OpenOffice (http://www.openoffice.org).
It does have change tracking, although I don't know how well it works if you move documents back and forth between OpenOffice and Word.