Quick Solutions (for Windows*):
1. (DirectShow-only) http://www.microsoft.com/windows/windowsmedia/format/codecdownload.aspx (which package you download depends on your version of windows and windows media player)
2. (VfW) http://www./msmpeg4 - similar file mirrored on our site here
3. Update your version of Windows Media Player http://www.microsoft.com/windows/windowsmedia/download/AllDownloads.aspx
* Mac users see next post below
More info:
MPG4: Microsoft MPEG-4 version 1
MP42: Microsoft MPEG-4 version 2
MP43: Microsoft MPEG-4 version 3
The story behind it...
From codeccorner.com
“It is widely accepted that Microsoft attempted to take control of the MPEG-4 standard by coming out with an AVI version of the codec prior to the MPEG consortium having voted a final standard. The code had to be completely reworked 3 times. Making matters worse, Microsoft refused to rename the files, and since they were not backwardly compatible even more distress was created."
Furthermore, Microsoft locked their codec so it could no longer be used to encode AVIs, but would only encode with Microsoft’s own ASF container. Then out of the woodwork people made cracks to re-enable AVI encoding (DivX 3 being the most famous; AngelPotion was another one, but a poor one). The cracks were just the Microsoft MPEG-4 version 3 codec with new names and AVI support.
MPG4, MP42, and MP43 can be found in both AVI and Windows Media containers. They should be included in any standard installation of Windows 2000 and and newer. If for some reason they were accidentally lost or deleted, any of the links at the top of the page should work. In Windows 98 (first edition), they will be installed if the proper boxes have been checked during installation. (If not, go to control panel, add/remove programs, windows setup, multimedia, and check the box netshow player - you may be asked to insert your windows 98 cd)
For those interested in encoding in MP43 (it’s still a fine codec) in AVI (perhaps because you are concerned about the legal status of DivX 3) get solution 2 at the top of the page. 
