The problem is that a lot of AVI files was - doing a short period (during a beta stage of Windows Media Tools) - encoded using the MPEG-4v3. However when Microsoft released the next version of the tools, MPEG-4v3 was no longer supported in AVI files.

If you have an AVI file encoded using MPEG-4v3 and your version of the MPEG-4v3 codec (MPG4C32.DLL) is newer than 4.00.0.3688 you cannot play the file. To be able to play it anyway, install this older version of codec.

How to install MPEG-4v3 Codec :

- Media Player 6.4 (or better) must be installed on your system. If you don't know what version you have, open Media Player and click Help menu and then About item. If necessary, download Media Player from M$ website.
- Uncompress the files from MPEG-4v3_codec_fix.rar archive, right-click on the mpeg4fix.inf file and select Install.
- You may encounter a prompt saying: "The software you are installing for this hardware... has not passed Windows Logo testing..."; click on the Continue Anyway button.

You have now installed MPEG-4, MPEG-4v2, and MPEG-4v3. If you still can't view the clip, reboot your system and check the installed codec. See Checking Installed VIDEO and AUDIO Codecs In Windows XP guide.