Try rolling back your X-fi card drivers to a older version
Not sure about that, you might give it a shot, but there is, on the other hand, a new (beta) driver that you might try as well on creative website, one specifically made for 64 bit Vista I think.
But before that try following:
1) disable onboard audio in bios
2) disconnect any USB devices including wireless network adapters etc. It depends how USB was implemented but sometimes it hogs the PCI bus
2) change PCI latency values (in BIOS)- experiment with either 16 and 64 or lower/higher syarting with the default 32. If there is no such option you can google for PCI latency timer tool.
4) put the X-Fi card into a different PCI slot - if you can
5) try that beta driver for X-Fi