Which program are you using to downld with & which one to preview with. I would recommend VLC for previewing files. It's an ugly player but it's very effective as it has in-built codecs. I suspect codecs "might" be your problem. Also some players don't behave well with incomplete video files. See links for VLC here: CODECs list and alternate players for Movies
BTW are you launching songs from your downld folder or from your incomplete folder? For songs be sure they're not corrupt: T3 Spammers; mp3 Bit-Rates Versus File Sizes

Some video downld tips:

1. Movie download issues

2. Learn to user Bitzi lookup to see if there's any feedback on the particular video file: How to use Bitzi Web Lookup ...

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Imagination is more important than knowledge. -- Albert Einstein
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
640 K [of computer memory] ought be enough for anybody. -- Bill Gates (1981)
The secret to creativity is knowing how to hide your sources. -- Albert Einstein
When facts don't fit the theory, change the facts. -- Albert Einstein
Religion keeps the poor from murdering the rich. -- Napoleon Bonaparte
The important thing is not to stop questioning. -- Albert Einstein
I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. -- Albert Einstein
"A composer is a guy who goes around forcing his will on unsuspecting air molecules, often with the assistance of unsuspecting musicians." -- Frank Zappa
"Tobacco is my favorite vegetable." / "May your sh*t come to life and kiss you on the face." / "Scientology, how about that? You hold on to the tin cans and then this guy asks you a bunch of questions, and if you pay enough money you get to join the master race. How's that for a religion?" / "Communism doesn't work because people like to own stuff." / "There are three things that smell of fish. One of them is fish. The other two are growing on you!" / "Jazz is not dead, it just smells funny." / "The creation and destruction of harmonic and 'statistical' tensions is essential to the maintenance of compositional drama. Any composition (or improvisation) which remains consonant and 'regular' throughout is, for me, equivalent to watching a movie with only 'good guys' in it, or eating cottage cheese." / "Remember there's a big difference between kneeling down and bending over." / "The most important thing to do in your life, is to not interfere with somebody else's life." / "Rock journalism is people who can't write, interviewing people who can't talk, in order to provide articles for people who can't read." -- Frank Zappa
"The music business is a cruel and shallow trench, a long plastic hallway where thieves and pimps run free and good men die like dogs. There's also a negative side." - Hunter S. Thompson
The shy can light the dull dark room with life when their eyes mirror their inner sunrise - LOTR