I can't open iTunes on my computer.

When I tried to turn on iTunes yesterday I got the error message, "The procedure entry point AVCFPlayerItemDurationChangedNotification could not be located in the dynamic link library AVFoundation CF.dll." Then, when I clicked OK, I got a message saying iTunes was improperly downloaded and I need to download it again. How have I been using it for three years if it was improperly downloaded? I don't want to lose my library. I have much of it saved on CD's but lately I've been backing up to a flash drive and someone told me today that won't reload to iTunes. What do I do?