Well i have Asus ....A7N8X /VM Nforce 2 and i seem to be able to use both onboard as well as my Creative Audigy 2 Zs card..and so can my friend who has an intelMOBO board and one Turtle Sound card...can use his onboard sound as well as Pci card...for a bit of DJ ...So i think its fairly easy to use both...
Why don't you try formatting the os ...or if not possible unistalling all the drivers and all and follow the instal in this process...
1) Direct X 9.0C
2) Then Via 4 in 1 Driver ( Latest one downloaded from net ..not one from MOBO Cd )
3) Then Graphic Card driver ..
4) Then Onboard sound Card Driver ...
5) And then Finally the Creative Card Driver....
Should solve the problem...i guess....Please disable these ports if you dont use themmm...com port 1 & 2 Parellel Ports and one of the onboard game Port...insted use Creatives game port if youhave to use...

Best Luck...