Since I don't use Vista, I haven't noticed this personally, but I just read an article talking about how Vista and 7 removed the DirectSound hardware support and emulate it in software. Creative has created Alchemy for free that intercepts the DirectSound calls and sends them to OpenAL APIs which has access to the hardware.
http://www.creative.com/soundblaster/alchemy/
I came across this while reading this article:
http://www.techradar.com/news/computing-components/upgrades/whatever-happened-to-pc-soundcards--668292
Maybe some of you were already aware of this, but I thought I'd share.
"Only the educated are free" ~Epictetus
"Imagination is more important than knowledge..." ~Einstein