In the event you download Music, Books, TV shows or Applications from the iTunes Store and find the files are unusable, you can simply delete and download them again via iTunes in the Cloud. The iTunes Store gives you the ability to download your past purchases without incurring additional charges as long as what you want to download is shown on your Purchased page.
To enable this feature, you must have the latest version of iTunes. It is available on the Apple website:
http://www.itunes.com/download
To learn how to download past purchases, check out this article:
Downloading past purchases from the App Store, iBookstore, and iTunes Store
http://support.apple.com/kb/HT2519
*IMPORTANT* Note: iTunes in the Cloud is not available for all content types in all countries. To see if your country supports redownloading past purchases, check out this article:
http://support.apple.com/kb/HT5085