Which version of Premiere Elements are you using and on what operating system?
What processor do you have in your computer and with how much RAM?
What you're describing usually indicates that your hardware isn't up to the task of editing high-def video. On the other hand, it could also be a mismatch of your video source and your project specs.
But in order to determine that, we'll need to know these details.
I have premiere 10.
I have a pc with 8gb ram, processor Intel(R) Core (TM) i 7 -2600 Cpu 3.40 GHz 3.40 Ghz and 64.bits operativssytem. Should be enough. And it has worked before and is still working on my other computer. After cleaning the computer i Couldn't read mts.files. Avi files work as before.
" After cleaning the computer i Couldn't read mts.files. "
That's an interesting problem. It sounds like you've somehow removed the codec! (I assume you've already tried uninstalling and reinstalling the software and reinstalling the most recent version of Quicktime, per the program's requirements.)
What did you do to clean your computer? Did you run some sort of registry cleaner program?
What video card/graphics card does your computer use - NVIDIA GeForce? Are you doing SD or HD projects? And, is your video card/graphics card driver version on an automatic update schedule?
When you refer to Preview Windows, is that the Edit area monitor or the Preview Window (with Set In and Set Out points) that opens when you double click your file in project media?
Your answer to those questions will point you to being involved in a serious known Premiere Elements 10/NVIDIA Ge Force issue. Please review and consider if any of this applies to your situation.
However, the part about the .avi file working and not the .mts file is not falling into place in this suggestion. However, the answers will be in the details.