Hi,
• Which application are you using to watch the videos?
• Are you trying to watch the videos saved on the computer or from a CD/DVD?
• Does it happen with all the videos?
• Is there any error message?
• How long are you facing the issue?
However, refer to “Play an audio or video file: frequently asked questions” http://windows.microsoft.com/en-US/windows-vista/Play-an-audio-or-video-file-frequently-asked-questions
Diana D
Microsoft Answers Support Engineer
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