WMM problem: clips freezing?

Okay, let's start from the very beginning. I downloaded a few short movies from YouTube and I put them into WMM in the .mpg format, but also in the .avi format, just in case. I just needed about 10 seconds of all of the videos I uploaded, but whenever I would put that small clip in the timeline and pressed 'play' on the preview of the video, the clip would freeze for 4-5 seconds (image AND sound). I tried trimming the clips, but nothing happened, the clips would still freeze whenever I played them. I tried downloading the videos again, but the same thing happened. I know it's a lame program, but I really have to get this done in WMM. Please, help!