Windows Media Player 11 on XP - codec for MP4?

Every time I try to play an MP4 video in Media Player (which WinXP has set as the default MP4 player), I get the message that the file is unable to be played.

A search on the error message taught me that I am missing a codec.

Microsoft puts out a patch to allow playing MP4 files, but this patch is for Win Media Player 7 to 9 and Windows 98, ME or 2000. I tried to install it anyway, but it refuses to install on Windows XP.

I can view these MP4 videos with Apple's free Quicktime, but I like Media Player much more.

Is there a (free?) way to get the codec for MP4 installed for Media Player?

Thanks,
Joe

P.S. First post on this forum. I admit, that I did not filter through all of the readme file, all of the FAQ section, or follow through on all of the pages that came up from my search result. I did a little, though (probably more than most do), but I wasn't able to locate an answer.