Go to the iTunes Store, log into your account, and click the Purchases link under the Quick Links. From there you should be able to re-download some or all of your purchased content. Note that not all content has been licensed for re-downloading in all countries at this time; you can see what content you can download here:
In addition, not all movie students have agreed, and not all movies are available from those studios that have, even in countries where movies are in general available.
For more information, see: