Cannot play any videos - iTunes freezes

Hey guys,

a few days ago I installed iTunes again. From that on I cannot play any videos. When I want to play a movie, iTunes just freezes and I have to stop it with CTRL+ALT+Del. When I start iTunes in Safe-Mode I'm able to play music videos, but not movies - iTunes freezes again.

When I open the videos manually in QuickTime they are working fine. So it is definetely an iTunes problem...

I tried a lot of things, for example reinstalling, changing DirectX settings in QuickTime and so on....

I hope someboy here is able to help me.