You must check if iviaudio.ax (InterVideo AC3 filter) exist on your computer (in %windir%/system32/ directory for WinNT, Win2k, WinXP or in %windir%/system/ directory for Win98, WinMe) - if exist type in RUN:
%windir%\system32\regsvr32 %windir%\system32\iviaudio.ax (WinNT, Win2k, WinXP),
or
%windir%\system\regsvr32 %windir%\system\iviaudio.ax (Win98, WinMe)
if not go to
http://www.intervideo.com or download Nimo Codec Pack and install ONLY InterVideo Audio Decoder (iviaudio.ax).