I have 2 drives, one with XP and just installed Windows 7 RC on the other. Exact same issue with Creative X-Fi Extreme Gamer - fine in XP but not even detected in 7.
Here's what I had to do to fix it:
go to
Creative Worldwide Support > X-Fi XtremeGamer
and download Creative Sound Blaster X-Fi series
driver 2.18.0013
Run the install. This doesn't fix the issue, but after a reboot it makes the card visible in Device Manager. Right click the device > update
driver > browse my computer > pick from list > Creative X-Fi Audio Processor (WDM). In my case this caused Device Manager to stop responding, but don't worry, just pretend it is all OK.
I was now able to go to Control Panel > Sound and Creative X-Fi was visible. Set as default and jobs a good'un.
Hope this helps someobody - be interested to hear other peoples experiences with it.