windows media audio codec missing?

everything begins with my windows media player 9.0[for xp]I tried to play a protected wma file, and something appeared,
>>The audio codec identified by the format tag 161 is required to play this file. To determine if this codec is available to download from the Web, click Web Help<<
of course, web help from ms is always usless.
then i tried DivX_311alpha, nimo[<-i’m not sure about the name], but nothing appears tobe working. That’s not over. After I followed these steps->
Open Control Panel|Sounds & Audio Devices|Hardware|Audio Codecs|Properties|Properties
I saw no “Windows Media Codecs”||||
even after I reintalled windows media player10
btw, after I reintalled wmp10, the error became->
>>Windows Media Player encountered an unknown error. This can occur when another program or operating system component encounters a problem but does not communicate the nature of the problem to the Player<<

How can I solve this? Thanks.