Start with the article below to troubleshooting your install of iTunes. One other thing I would try is to download a new install file from Apple’s website and try to install that.
Thanks so much for your reply and for the links! I've tried everything but i didn't succeed, because i can't delete the old version of iTunes it always says that the installation source for this product is not available, make sure that the source exists and you can access it. But i can't find the source because everything else is gone...do i have to restore my HD and make everything new?