how to convert a .dat video format to .avi video format

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hi, I have a cd which contains a video in a dat format. I want to convert the video to avi format. Can anybody suggest a free software than can do this---change a dat format video to avi format video. I have tried several free software but it either doesnt work or it just converts 50% of the video--- it says I have to buy the software in order for the whole video file to be converted. help will be greatly appreciated.