Might be a silly question but have you enabled the 5.1 option in windows as well as Creatives' program? Might not hurt to double check:
Start--->Control Panel --->Sounds and Audio Devices--->VolumeTab (SpeakerSettings section)--->Advanced button.
By the way you may not get full surround sound from Power DVD without buying their "audio pack". (It's an "added feature" to the "full" version that came with my DVD ROM.)
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