why won't movies play to end

Hello,

I run Windows 7 on my Toshiba laptop, and recently have not been able to complete watching downloaded movies.. (fully downloaded from iTunes).... This has formerly been a pleasant way for me to watch movies, and is not working.

How can I fix ? I have very little stored on my laptop and have about half my memory unused.... ANY Help will be appreciated !

Thank you, in advance !

(p.s. does iTunes refund movie purchases if you can't actually watch them ?)