I had this problem too. I was trying to run an old 32-bit dll in a 64 bit system. I got it working by copying the .dll to the C:\Windows\SysWoW64\ directory and running this:
%systemroot%\SysWoW64\regsvr32 "C:\Windows\SysWoW64\thenameofyourdll.dll"
And also enabling IIS to run 32 bit apps