Youtube videos stopping/lagging in my powerpoint presentations

My operating system is Windows 7 and I'm not a computer smart as I want to be. I'm preparing a powerpoint presentation for Veteran's Day with Office Pro 2010 and I use youtube videos in them. When I download them I convert them to Windows Media Video (V.7WMV) so I can play my presentation without needing the internet and it prompts me to view the video after it's done and it runs fine. But when I insert it and I play the videos they are stopping/lagging prior to the video finishing. Is it my video or audio card configured wrong? What can I do? Please help me!!!!!!

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