Vista - SPDIF Passthrough

Hi,

I've having a nightmare getting 5.1 surround sound from a Divx AVI
file which is encoded with the AC3 codec on Windows Vista x64. The
file works perfectly on my old XP Media Center but in vista I just get
2 channel stereo.

The strange thing is that when I play a DVD in media center, Power DVD
7, or Media Player then everything is OK and I get the DTS or Dolby
sent straight to my AV Receiver. Also when I go into the SPDIF
configuration tool under sound devices control panel the test of Dolby
and DTS work fine. So it seems as if its just when I play AVI files
with the AC3 codec.

I've installed a Vista 64 Codec pack with ffdshow which did not solve
the problem. I have also tried AC3 filter which again did not solve
the problem. They both seem to send a 2 channel signal to my receive,
however, the receiver does indicate that it is a dolby digital signal.

I have Windows Vista x64 Home Premium (retail version) with a Creative
X-FI Music sound card and the optical out module.

Has any one had similar problems and been able to solve it?

Cheers,
Gazza