What can I do if Prism can't convert my video file, or the converted file is played incorrectly?

You can try to force Prism to use it's built-in video codec (libavcodec) which supports a number of different video file formats and compressions.

Go to the Options -> Conversions and select "Use libavcodec before DirectShow".

For .avi files, make sure the video file is playing fine in Windows Media Player. If it is not, you can try to open it in Prism and look at the "Other Information" field. There should be a "Codec" name. Try to download and install the required codec. Then try to convert it again.