Using PE 10 weekly, no problems until updating to Windows 8.1. Not sure if that is the issue, but after updating I try to compose a video using the same Mp4 HD Clips from my Panasonic FZ200 that I always use, can't view any video, only get audio. Tried multiple project settings including letting PE choose for me. Updated QT.
Any idea where to start here?
My first guess would be that you have an nVidia card/chip (GPU). Is that correct?
Second guess is that you have a very recent nVidia video driver. Is that also correct?
If so, then you will want to roll-back your nVidia video driver, per this thread: http://forums.adobe.com/thread/1317675?tstart=0
Good luck, and if you do not have an nVidia GPU, please tell us more about your computer.