Over on the creative boards, several people seem to have found success with getting their x-fi's to work with 5.1 sound, mode switching, etc...using the newest xp32 drivers:
http://forums.creative.com/creativelabs/board/message?board.id=Vista&message.id=227&page=1
Might be something to try. This is for vista32, btw...vista64 is still OOL...

x-fi drivers - possible fix?
Windows Vista
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