Vista - Default Sound Playback Device Changes itself

This in Vista Ultimate x64. I have three playback devices listed: Speakers
(Createive SB X-Fi), SPDIF Out (Creative SB X-Fi), and Speakers (USB Audio
Device --actually a Logitech EasyCall mike/speaker for VoIP calls). The
problem is, I keep setting the default to Speakers (Creative SB X-Fi). . .and
yet a few hours or days later when I try to play a youtube video or something
I get no sound and when I look the default is back to SPDIF.

I don't think this is Creative's fault, as the same thing was happening when
I had my onboard sound enabled instead --I'd have the default at speakers and
when it didn't work I'd check and it'd be SPDIF again without any
intervention by me.

Help?