J'ai eu le même problème et j'ai trouvé la solution, suis ça :

Found out that WVC1DMOE.DLL works only under Windows XP/2003/Vista with WMP11 installed.
To make this work under Windows 98/98 SP1/98 SE/ME, please follow these updated instructions:
http://www.mdgx.com/wmp.htm#ENC

WVC1 Codec [899 KB]:
http://www-pc.uni-regensburg.de/systemsw/W...dia/wvc1dmo.exe­
How to install WMV9AP VC-1 Codec on Windows 98/NT4/ME:
1. Create C:\TEMP (example) from a DOS box:
MD C:\TEMP
2. Download wvc1dmo.exe (see above) into C:\TEMP .
3. Run:
C:\TEMP\wvc1dmo.exe /C /Q /T:C:\TEMP
4. Copy WVC1DMOD.DLL from C:\TEMP into %windir%\SYSTEM [%windir% = usually C:\WINDOWS]:
COPY/Y C:\TEMP\WVC1DMOD.DLL %windir%\SYSTEM
5. Run this command from a DOS box:
REGSVR32 %windir%\SYSTEM\WVC1DMOD.DLL
6. Copy + paste text between cut & paste lines below in Notepad -> save as WVC1_9X.REG (example) -> (double)-click on WVC1_9X.REG to merge into Registry:

-----Begin cut & paste here-----
REGEDIT4

[HKEY_LOCAL_MACHINE\Software\CLASSES\Windows Media\WMSDK\VideoEncode]
"WVC1"=""
[HKEY_LOCAL_MACHINE\Software\CLASSES\Windows Media\WMSDK\VideoDecode\WVC1]
"DllName"="WVC1DMOD.DLL"

------End cut & paste here------

7. Delete C:\TEMP .

Je l'ai trouvé sur ce site : http://www.msfn.org/board/WVC1-Codec-t100829.html