I'd try reinstalling.
If that doesn't work, more info will be needed. We don't even know what version of Windows you're running! What versions of AE you've had before, etc.
Do you have GoPro Studio installed? Have you ever installed CineForm codecs? Have you installed any third-party codec packs (such as K-Lite)?
I'm asking because all of these things might cause problems depending on your situation. (Third-party codec packs can cause LOTS of odd/terrible problems...)