You need to be more specific! What version of AE? Running the trial? Where do those files come from? Are there other programs on the system that may have a hold on the relevant CoDecs? What are your prefs for hardware acceleration in AE? What system specs? Could be that things are busted on your system, could be that you are simply using crappy, non-standard compliant video from a cheap digital photo camera...
Try searching for H.264, too, we see this same post often.
My workflow never uses highly compressed codecs like H.264. I awlays unpack everything and tronscode to fully frame-based codecs like ProRes422.