As I understand it the difference between 1394 and 1394b is the speed - 1394 is 400 - 1394b is 800.
The cable shouldn't be bad, but I have seen stranger things. There are two firewire ports on the back of the multimix, have you tried them both? The 6 to 4 pin cable is for a laptop, a6 to 6 pin is to receive power from a desktop motherboard.
Look in the device manager to see which firewire port you have. If it is a 1394b then there is a Windows update for XP that increases the speed back to where it should be. There is a link in this thread to it. If it is a 1394 port then you don't need it.
Disable the onboard sound card through the device manager. Or in the BIOS if you can get into it.
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randy
I have never loaded Cubase that came with it, I have always used Sonar.
Check for an IRQ conflect while you are in Device Manager or in the System Info.