How to Fix Error -50 on iTunes
ITunes can occasionally return inscrutable error messages -- in particular, the dreaded "Error (-50)" message when you download music or movies from the iTunes store. You get this error for a couple of different reasons: generally, it means you have a firewall or virus program blocking the download, and need to allow iTunes to access the Internet. Another reason pertains only to videos: the downloaded cache files for the video are causing the problems. To resolve it, you need to delete the files.
Open your antivirus/firewall software.
Set the software to allow data from these domains: itunes.apple.com, ax.itunes.apple.com, albert.apple.com or gs.apple.com. The exact process will vary depending on the software you have.
Attempt to download the purchased music or video again.
Video Temp Files
Browse to your computer's downloads directory. For Windows 7 machines, it is typically "\Users\username\My Music\iTunes\iTunes Media\Downloads\" -- replace "username"with your username.
Find the folder containing the temp file. In Windows 7, it is named after the video and has a ".tmp" extension.
Drag the .tmp folder to the Recycle Bin.
Select "Store," then "Check for Available Downloads."
Enter your account name and password when prompted.
Click "Check." Your file starts to download again.