Try the SysWow64 directory, i had trouble regsvr32-ing the 32bit mpcvideodec.ax filter, then i tried sticking it in that directory and it worked.
and i agree with XP SP3 upgrade!! ;-)
I didn't found such directory (I've tried "syswow64" and "*syswow*)
Maybe it is possible to track where this "graphstudio" seeks for this dll file and stuck it there?
I'll thing hardly about upgrading to XP