Can't play videos in Itunes?

When I click on a movie in itunes, the screen comes up but it automatically puts the slider to the end of the movie. If I move the slider over to another part of the movie, the picture at that instant pops up, but the movie will not play. I can run the files separate from Itunes when I use Quicktime player, but I can't run anything within Itunes.

I'm running Windows 7, 64 bit.

Any help would be greatly appreciated.