In order to use software such as Ableton Live or Traktor to mix "live" you need to have a sound output channel and a monitoring channel for cuing up the next track, etc.
If you have a soundcard that has more than one output channel then this isn't an issue, but if as in my case you have more than one soundcard installed you are stuffed as AL/Traktor won't allow you to select different soundcards for output.
Mac OS allows the aggregation of more than one soundcard into one effective device, but Windows (XP or Vista) has no facility to allow this. Linux will also allow this but the audio software isn't there to make this worthwhile anyway.
Step up
Asio4all. This clever Asio driver allows you to channel output to different soundcards. Although my first experiments with this were a disaster - terrible sound quality/doubling of bpm!/slow response, a bit of fiddling with settings has come up with a solution that seems to work really well.
I've not set this up in AL yet but here are my settings in Traktor Pro:
For some reason I had to use output pair 5&6 from the creative card, 1&2 / 3&4 gave no sound.
It is a good idea to get the buffer offset as low as you can.
This solution will never be as good as having a multi-output soundcard but it's better than nothing.
Hope that helps if you have similar problems.