double clicking not working (videos)

[Computerb]

You don't mention what happens when you double click the file. The first thing I would check would be the 'file associations'. From Windows Explorer click Tools, Folder Options, File Types. Scroll to the file extension you want to associate and make sure the program you want to open it with is selected. For example, if it is an .avi file I use VLC media player. You preference is Windows Media Player.