just get a soundcard with spdif output
it can be a cheap one, the digital out basically bypasses the card, and outputs PCM directly out, and you can also bypass the card and output AC3 audio from dvd or ac3 encoded video files directly out to the receiever for dolby digital and dts
this will NOT output computer sounds in surround sound, tho, such as games that are in surround sound, those need to be decoded by the card itself, and will not be properly sent to the external receiever