What Keeps Turning My SPDIF On In Vista?

Hi All,
I’m running Vista Business 64 and have a discreet sound card using analog in and out. Every now and then, something turns the Windows spdif on making it the default and leaves me without sound. It only happens upon start-up and once in a blue moon but I just can’t put my finger on it. It took me a while to realize what was happening when I had no sound on start-up and had to restore to an earlier date to get sound back as a reboot did nothing. After many times looking at Windows sound page, and it showing that everything was working properly, I finally saw that little green check mark showing that the spdif had been selected as the default sound. I had totally disabled the spdif by selecting it, going to properties and then clicking “Don’t Use This Device (disable)”. I then select my sound card as the default sound source and it will work fine but every now and then, the process will reverse itself and choose the spdif as the default.
Does anyone have any idea what causes this? It doesn’t happen often but is an inconvenience when it does.